Back out "cleanup feature flag enableFabricCompleteRootInCommitPhase" (facebook#49565)

Summary:
Pull Request resolved: facebook#49565

Original commit changeset: 6ed54f0fab45

Original Phabricator Diff: D69473992

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D69924542

fbshipit-source-id: 6438d1f6e21728de662a060893b0f08f9285be73

Author: Jack Pope

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

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

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

@@ -185,6 +185,8 @@ 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'});