Pause UIScene migration (#170457)

UIScene migration is not yet enforced by Apple so we have some more time
to handle this. Let's pause on auto-migrating projects.

## 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].

<!-- Links -->
[Contributor Guide]:
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

packages/flutter_tools/lib/src/ios/mac.dart

@@ -42,7 +42,6 @@ import 'migrations/project_build_location_migration.dart';
 import 'migrations/remove_bitcode_migration.dart';
 import 'migrations/remove_framework_link_and_embedding_migration.dart';
 import 'migrations/uiapplicationmain_deprecation_migration.dart';
-import 'migrations/uiscenedelegate_migration.dart';
 import 'migrations/xcode_build_system_migration.dart';
 import 'xcode_build_settings.dart';
 import 'xcodeproj.dart';
@@ -153,7 +152,6 @@ Future<XcodeBuildResult> buildXcodeProject({
     IOSDeploymentTargetMigration(app.project, globals.logger),
     XcodeProjectObjectVersionMigration(app.project, globals.logger),
     HostAppInfoPlistMigration(app.project, globals.logger),
-    UISceneDelegateMigration(app.project, globals.logger),
     XcodeScriptBuildPhaseMigration(app.project, globals.logger),
     RemoveBitcodeMigration(app.project, globals.logger),
     XcodeThinBinaryBuildPhaseInputPathsMigration(app.project, globals.logger),

packages/flutter_tools/lib/src/ios/migrations/uiscenedelegate_migration.dart

@@ -1,94 +0,0 @@
-// Copyright 2014 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 'package:meta/meta.dart' show visibleForTesting;
-import 'package:xml/xml.dart';
-
-import '../../base/file_system.dart';
-import '../../base/project_migrator.dart';
-import '../../xcode_project.dart';
-
-const String _manifestKey = 'UIApplicationSceneManifest';
-const String _storyboardKey = 'UIMainStoryboardFile';
-
-String _addition(String storyboard) => '''
-<key>UIApplicationSceneManifest</key>
-<dict>
-  <key>UIApplicationSupportsMultipleScenes</key>
-  <false/>
-  <key>UISceneConfigurations</key>
-  <dict>
-    <key>UIWindowSceneSessionRoleApplication</key>
-    <array>
-      <dict>
-        <key>UISceneClassName</key>
-        <string>UIWindowScene</string>
-        <key>UISceneDelegateClassName</key>
-        <string>FlutterSceneDelegate</string>
-        <key>UISceneConfigurationName</key>
-        <string>flutter</string>
-        <key>UISceneStoryboardFile</key>
-        <string>$storyboard</string>
-      </dict>
-    </array>
-  </dict>
-</dict>
-''';
-
-/// Update Info.plist.
-class UISceneDelegateMigration extends ProjectMigrator {
-  UISceneDelegateMigration(IosProject project, super.logger)
-    : _infoPlist = project.defaultHostInfoPlist;
-
-  final File _infoPlist;
-
-  @override
-  Future<void> migrate() async {
-    if (!_infoPlist.existsSync()) {
-      logger.printTrace('Info.plist not found, skipping host app Info.plist migration.');
-      return;
-    }
-
-    processFileLines(_infoPlist);
-  }
-
-  @visibleForTesting
-  static Map<String, XmlElement> extractPlistDict(XmlElement dict) {
-    final Map<String, XmlElement> keyValues = <String, XmlElement>{};
-    String? key;
-    for (final XmlElement child in dict.childElements) {
-      if (key != null) {
-        keyValues[key] = child;
-        key = null;
-      } else {
-        key = child.innerText;
-      }
-    }
-    return keyValues;
-  }
-
-  @override
-  String migrateFileContents(String fileContents) {
-    try {
-      final XmlDocument read = XmlDocument.parse(fileContents);
-      final XmlElement readPlist = read.childElements.first;
-      final XmlElement readDict = readPlist.childElements.first;
-      final Map<String, XmlElement> keyValues = extractPlistDict(readDict);
-
-      if (!keyValues.containsKey(_manifestKey)) {
-        if (keyValues.containsKey(_storyboardKey)) {
-          final String storyboard = keyValues[_storyboardKey]!.innerText;
-          final String xmlAddition = _addition(storyboard);
-          final XmlDocumentFragment fragment = XmlDocumentFragment.parse(xmlAddition);
-          for (final XmlNode node in fragment.children) {
-            readDict.children.add(node.copy());
-          }
-        }
-      }
-      return '${read.toXmlString(pretty: true)}\n';
-    } on Exception catch (_) {
-      return fileContents;
-    }
-  }
-}