Inline true for fabricEnabled in DefaultNewArchitectureEntryPoint (#56882)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56882

Now that `fabricEnabled` always returns `true`, delete the `privateFabricEnabled` backing field. Remove the redundant assignment in `load()` and inline `true` at its remaining usage in `isConfigurationValid` from `loadWithFeatureFlags`.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: alanleedev, fkgozali

Differential Revision: D105639307

fbshipit-source-id: 0c229ece1b50965cf9b6dcf8ec744d5373f8a91d