expose ref to <Modal />'s native component (#49859)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49859

changelog: [internal]

this will be used for testing purposes in Fantom.

This diff also cleans up `ModalInjection.js` which is unused.

Reviewed By: rubennorte

Differential Revision: D69883384

fbshipit-source-id: 4ff278ef174c95fd5a93f70519f14c5190a5ada5