RN: Cleanup alwaysFlattenAnimatedStyles Flag (#52682)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52682

Cleans up the `alwaysFlattenAnimatedStyles` feature flag.

This was already unused starting with {D77314904}.

Changelog:
[Internal]

Reviewed By: mdvacca

Differential Revision: D78497554

fbshipit-source-id: 8bfbe6485812a675cdf3e78612347457f45ada90

packages/react-native/scripts/featureflags/ReactNativeFeatureFlags.config.js

@@ -708,17 +708,6 @@ const definitions: FeatureFlagDefinitions = {
 
   jsOnly: {
     ...testDefinitions.jsOnly,
-    alwaysFlattenAnimatedStyles: {
-      defaultValue: false,
-      metadata: {
-        dateAdded: '2025-06-02',
-        description:
-          'Changes `Animated` to always flatten style, fixing a bug with shadowed `AnimatedNode` instances.',
-        expectedReleaseValue: true,
-        purpose: 'experimentation',
-      },
-      ossReleaseStage: 'none',
-    },
     animatedShouldDebounceQueueFlush: {
       defaultValue: false,
       metadata: {

packages/react-native/src/private/featureflags/ReactNativeFeatureFlags.js

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<d30fc4e36ad600353c9b9098fd71430a>>
+ * @generated SignedSource<<742172e146c6eeb22c2b5058058b1b83>>
  * @flow strict
  * @noformat
  */
@@ -29,7 +29,6 @@ import {
 
 export type ReactNativeFeatureFlagsJsOnly = $ReadOnly<{
   jsOnlyTestFlag: Getter<boolean>,
-  alwaysFlattenAnimatedStyles: Getter<boolean>,
   animatedShouldDebounceQueueFlush: Getter<boolean>,
   animatedShouldUseSingleOp: Getter<boolean>,
   avoidStateUpdateInAnimatedPropsMemo: Getter<boolean>,
@@ -120,11 +119,6 @@ export type ReactNativeFeatureFlags = $ReadOnly<{
  */
 export const jsOnlyTestFlag: Getter<boolean> = createJavaScriptFlagGetter('jsOnlyTestFlag', false);
 
-/**
- * Changes `Animated` to always flatten style, fixing a bug with shadowed `AnimatedNode` instances.
- */
-export const alwaysFlattenAnimatedStyles: Getter<boolean> = createJavaScriptFlagGetter('alwaysFlattenAnimatedStyles', false);
-
 /**
  * Enables an experimental flush-queue debouncing in Animated.js.
  */