Ship refactor of XHR, FileReader and WebSocket to use the built-in EventTarget implementation (#50485)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50485

Changelog: [internal]

This cleans up the experiment to test the new implementations of `XMLHttpRequest`, `FileReader` and `WebSocket` using the built-in `EventTarget` definition.

This effectively replaces the legacy implementations with the ones using the built-in `EventTarget` class in stable.

Reviewed By: yungsters

Differential Revision: D68625225

fbshipit-source-id: 7ccefe5d8612baca283146cdbca7f7f1d51b4ffa