Remove hermes-inspector-msggen (#48465)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48465

Removes the `packages/hermes-inspector-msggen` workspace. Since https://github.com/facebook/react-native/pull/39300, this is no longer referenced in React Native, and is now part of the Hermes repo.

Changelog: [Internal]

Reviewed By: cortinico, hoxyq

Differential Revision: D67791612

fbshipit-source-id: 73da135b264d8df632fefe87cc4e3101075ae98c