Remove build package.json dirty state check (#55420)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55420

After https://github.com/facebook/react-native/pull/54857 (use of the "publishConfig" field), this check is no longer necessary, since `yarn build` will no longer make changes to any checked in files.

In the (expectedly) rare case that `yarn build --prepack` changes are committed (only intended for CI), Flow will also fail independently.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D92397786

fbshipit-source-id: 872cc91b62295cdb6d8c2799cffb8a4cadc4106e