Remove index.js.flow shim from Node packages (#49461)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49461

Flow now supports Package Exports 🎉. This means we can delete the compatiblity pattern in each of our build-enabled Node.js packages.

This simplifies the internal package structure needed to support Flow while developing from source in the monorepo — no prod impact.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D69741143

fbshipit-source-id: 070715cb6beb00eb393186dbf95856ceb87fabef