[mergebot] Remove Comment Validator for Reverting (#80866)
Since migrating to a CLI parser, we don't want to duplicate checks for validating the revert command in both the merge bot and in trymerge.
Also there's no similar regex for the merge command so just making it consistent with that.
Addresses https://github.com/pytorch/test-infra/issues/361
Test Plan:
Check that tests pass
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80866
Approved by: https://github.com/suo, https://github.com/malfet
