Fix duplication across and rncore and FBReactNativeComponentSpec (#43385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43385

`rncore` and `FBReactNativeComponentSpec` contain the same symbols, which leads to conflicts when we try to merge them into a single shared library. Cleanup the duplication and standardize on `FBReactNativeComponentSpec` everywhere. I've left the Android OSS targets as is, to avoid breaking deps.

Changelog: [Internal]

Reviewed By: cortinico, dmytrorykun

Differential Revision: D54630694

fbshipit-source-id: 75cb961ded9fd75508755c0530e29409fef801cf