macOS: Delete unused FlutterRenderBackingStore (#168159)

All usages of this code were removed in
ed9d9bf372ba9ebd97446fb8a8f8b03f0f42986a during the great macOS renderer
refactor of 2022.

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -52907,8 +52907,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterApp
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegate.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegateTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegate_Internal.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponderTest.mm + ../../../flutter/LICENSE
@@ -55920,8 +55918,6 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDe
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegate.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegateTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppLifecycleDelegate_Internal.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponderTest.mm

engine/src/flutter/shell/platform/darwin/macos/BUILD.gn

@@ -60,8 +60,6 @@ source_set("flutter_framework_source") {
     "framework/Source/AccessibilityBridgeMac.mm",
     "framework/Source/FlutterAppDelegate.mm",
     "framework/Source/FlutterAppLifecycleDelegate.mm",
-    "framework/Source/FlutterBackingStore.h",
-    "framework/Source/FlutterBackingStore.mm",
     "framework/Source/FlutterChannelKeyResponder.h",
     "framework/Source/FlutterChannelKeyResponder.mm",
     "framework/Source/FlutterCompositor.h",

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h

@@ -1,29 +0,0 @@
-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef FLUTTER_SHELL_PLATFORM_DARWIN_MACOS_FRAMEWORK_SOURCE_FLUTTERBACKINGSTORE_H_
-#define FLUTTER_SHELL_PLATFORM_DARWIN_MACOS_FRAMEWORK_SOURCE_FLUTTERBACKINGSTORE_H_
-
-#import <Cocoa/Cocoa.h>
-#import <Metal/Metal.h>
-
-/**
- * Interface for backing store handles. Typically contain references to the buffers that
- * are handed by the `FlutterView` to the `FlutterRenderer`.
- */
-@interface FlutterRenderBackingStore : NSObject
-
-/**
- * MTLTexture referenced by this backing store instance.
- */
-@property(nonnull, nonatomic, readonly) id<MTLTexture> texture;
-
-/**
- * Initializes a backing store with the specified MTLTexture.
- */
-- (nonnull instancetype)initWithTexture:(nonnull id<MTLTexture>)texture;
-
-@end
-
-#endif  // FLUTTER_SHELL_PLATFORM_DARWIN_MACOS_FRAMEWORK_SOURCE_FLUTTERBACKINGSTORE_H_

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm

@@ -1,17 +0,0 @@
-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h"
-
-@implementation FlutterRenderBackingStore
-
-- (instancetype)initWithTexture:(id<MTLTexture>)texture {
-  self = [super init];
-  if (self) {
-    _texture = texture;
-  }
-  return self;
-}
-
-@end