Remove --enable-impeller-{backend} flags (always True). (flutter/engine#51402)

Closes https://github.com/flutter/flutter/issues/142725.

engine/src/flutter/ci/builders/local_engine.json

@@ -11,7 +11,6 @@
         "--android",
         "--android-cpu=arm64",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "android_debug_arm64",
@@ -31,7 +30,6 @@
         "--android",
         "--android-cpu=arm64",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "android_profile_arm64",
@@ -51,7 +49,6 @@
         "--android",
         "--android-cpu=arm64",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "android_release_arm64",
@@ -69,7 +66,6 @@
         "--runtime-mode",
         "debug",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "host_debug",
@@ -87,7 +83,6 @@
         "--runtime-mode",
         "profile",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "host_profile",
@@ -105,7 +100,6 @@
         "--runtime-mode",
         "release",
         "--no-stripped",
-        "--enable-impeller-vulkan",
         "--no-lto"
       ],
       "name": "host_release",
@@ -115,4 +109,4 @@
       }
     }
   ]
-}
+}

engine/src/flutter/ci/builders/mac_host_engine.json

@@ -28,7 +28,6 @@
                 "--no-lto",
                 "--prebuilt-dart-sdk",
                 "--build-embedder-examples",
-                "--enable-impeller-vulkan",
                 "--use-glfw-swiftshader"
             ],
             "name": "host_debug",
@@ -158,8 +157,6 @@
                 "--no-lto",
                 "--prebuilt-dart-sdk",
                 "--build-embedder-examples",
-                "--enable-impeller-vulkan",
-                "--enable-impeller-opengles",
                 "--use-glfw-swiftshader"
             ],
             "name": "host_release",

engine/src/flutter/impeller/docs/android_validation_layers.md

@@ -9,7 +9,6 @@ Example:
 ```sh
 flutter/tools/gn \
   --runtime-mode=debug \
-  --enable-impeller-vulkan \
   --enable-vulkan-validation-layers \
   --no-lto \
   --unoptimized \

engine/src/flutter/impeller/docs/renderdoc_frame_capture.md

@@ -8,7 +8,7 @@
 ```bash
 # In your $ENGINE_SRC folder, do:
 
-./flutter/tools/gn --unopt --enable-impeller-vulkan
+./flutter/tools/gn --unopt
 ninja -C out/host_debug_unopt/
 ```
 
@@ -17,9 +17,10 @@ ninja -C out/host_debug_unopt/
 ![Launch App](assets/renderdoc_frame_capture/launch-app.png)
 
 4. Fill out the following parameters, and click launch:
-  - executable: `$ENGINE_SRC/out/host_debug_impeller_vulkan/impeller_unittests` (expand `ENGINE_SRC`).
-  - args: `--gtest_filter="*CanDrawRect/Vulkan*" --enable_playground`
-  - working dir: `$ENGINE_SRC` (expand `ENGINE_SRC`)
+
+- executable: `$ENGINE_SRC/out/host_debug_impeller_vulkan/impeller_unittests` (expand `ENGINE_SRC`).
+- args: `--gtest_filter="*CanDrawRect/Vulkan*" --enable_playground`
+- working dir: `$ENGINE_SRC` (expand `ENGINE_SRC`)
 
 5. For the frame you wish to capture, press `F12`, you will now be able to see the frame capture and inspect the state.
 

engine/src/flutter/tools/gn

@@ -1237,20 +1237,6 @@ def parse_args(args):
   )
 
   # Impeller flags.
-  parser.add_argument(
-      '--enable-impeller-vulkan',
-      default=True,
-      action='store_true',
-      help='Enable the Impeller Vulkan backend.'
-  )
-
-  parser.add_argument(
-      '--enable-impeller-opengles',
-      default=True,
-      action='store_true',
-      help='Enable the Impeller OpenGL ES backend.'
-  )
-
   parser.add_argument(
       '--prebuilt-impellerc',
       default=None,