Remove leftover fields from root package.json (#44076)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44076

Removes fields in the root `package.json` manifest left over from the monorepo migration. `react-native/monorepo` is not a package published to npm, but is a root project configuration for the monorepo. Therefore it **doesn't need**:

- npm metadata fields (or even a `name` or `version` — I'm leaving these included due to 1/ references in fbsource, 2/ some non-Yarn tooling may complain).
- Fields used by tooling that are present in packages/react-native: `jest-junit`, `types`.
- A `peerDependency` on `react` (again, present in packages/react-native/package.json).

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D56134668

fbshipit-source-id: bc3449eb4c122eb5d885fabda9af7d19bb71faff