remove dead proto_convert library (#97598)
This has no code, only a collection of headers. Just make sure the
only thing that includes it still builds.
Differential Revision: [D44395700](https://our.internmc.facebook.com/intern/diff/D44395700/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D44395700/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97598
Approved by: https://github.com/PaliC
