Remove enableFabricRenderer() call from DefaultNewArchitectureEntryPoint (#56854)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56854

The `enableFabricRenderer()` flag is being deleted; it always returns true on the canary release stage. In `loadWithFeatureFlags`, replace both `featureFlags.enableFabricRenderer()` reads with the literal `true` for `privateFabricEnabled` and `privateConcurrentReactEnabled`.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D105229919

fbshipit-source-id: 9ec8a66addb198be123ac9f1a99d851cf88aa0c6