Remove useTurboModules() overrides from downstream consumers (#56899)

Summary:
Pull Request resolved: #56899

The `useTurboModules()` virtual method is going to be removed from `ReactNativeFeatureFlagsDefaults` and its default already flipped to `true`. This diff removes the now-stale `useTurboModules() override` methods from downstream `ReactNativeFeatureFlagsDefaults` subclasses so they compile cleanly once the parent method is gone.

Also drops the now-unused `enableFabric_` and `enableTurboModule_` private fields from the Catalyst wrapper override (`FBCWFeatureFlagsSetup.mm`) — they were only referenced by the conditional `useTurboModules` override removed here.

Updates `fbobjc/Apps/Instagram/Airwave/CLAUDE.md` to drop `useTurboModules` from the Airwave override doc, matching the override file.

Behavior is unchanged because every override returned `true` (the value the runtime always takes at this point) and the underlying default is now `true` as well.

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D105718206

fbshipit-source-id: cb1ebd9d4069368273f8c0d703956cd72e0ac644

Author: mdvacca

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlagsOverridesOSSStable.h

@@ -17,10 +17,6 @@ class ReactNativeFeatureFlagsOverridesOSSStable : public ReactNativeFeatureFlags
   {
     return true;
   }
-  bool useTurboModules() override
-  {
-    return true;
-  }
   bool useNativeViewConfigsInBridgelessMode() override
   {
     return true;