Remove enableFabricRenderer() call from ReactInstanceManager (#56855)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56855

The `ReactNativeNewArchitectureFeatureFlags.enableFabricRenderer()` flag is being deleted; it always returns true on the canary release stage. In `createRootView`, inline `setIsFabric(true)` and remove the now-unused import.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D105230246

fbshipit-source-id: 59d209ef712be7bf486d9110f14c4de4bc3e500e