Inline true in ReactDelegate.isFabricEnabled initializer (#56859)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56859

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

The field itself is preserved because it can be set by the deprecated `(activity, reactNativeHost, appKey, launchOptions, fabricEnabled)` constructor that lets callers override the default.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D105231216

fbshipit-source-id: c182ee3b8e045961b72349ffd7fbf4a4f5dca63f