cleanup shipped flag enableFabricCompleteRootInCommitPhase

Summary:
Flag was removed in {D74196673}, this cleans up the dynamic flag.

bypass-github-export-checks

Changelog: [internal]

Reviewed By: jackpope

Differential Revision: D74493584

fbshipit-source-id: 6ade542b7e1bd9c4367b7dbef4d2f1ec2d79d0df

Author: kassens

packages/react-native-fantom/runtime/mocks/ReactNativeInternalFeatureFlags.js

@@ -9,6 +9,4 @@
  * @oncall react_native
  */
 
-module.exports = {
-  enableFabricCompleteRootInCommitPhase: true,
-};
+module.exports = {};

packages/react-native/Libraries/ReactNative/__tests__/ReactFabric-Suspense-itest.js

@@ -186,8 +186,6 @@ describe('Suspense', () => {
     expect(resolveFunction).toBeNull();
   });
 
-  // TODO(T207868872): this test only succeeds with enableFabricCompleteRootInCommitPhase enabled.
-  // enableFabricCompleteRootInCommitPhase is hardcoded to true in the testing environment.
   it('shows stale data while transition is happening', () => {
     cache.clear();
     cache.set(SquareId.Green, {color: 'green'});