Inline true in ReactActivityDelegate.isFabricEnabled() (#56857)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56857

The `ReactNativeNewArchitectureFeatureFlags.enableFabricRenderer()` flag is being deleted; it always returns true on the canary release stage. The default body of `ReactActivityDelegate.isFabricEnabled()` reads the flag — change it to `return true` directly.

The method itself is preserved because it is a polymorphic `protected` hook that subclasses (e.g., `DefaultReactActivityDelegate`) override to provide per-activity behavior.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D105230961

fbshipit-source-id: 7acccbdca62e10022258523fd2a6eeb523b96273