Use React Native built-in definitions for Event and EventTarget in Fantom (#53362)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53362

Changelog: [internal]

This just replaces the polyfills for `Event` and `EventTarget` that we're defining inline in Fantom with the implementations that already exist in RN.

Reviewed By: javache

Differential Revision: D80612067

fbshipit-source-id: 047c8f12cbb1f4afea2d05a5a1235d9dff2e25f9