[PR Workflow] Suppression labels should be cleaned up when suppressions are removed from source

See https://github.com/Azure/azure-rest-api-specs/pull/25817#issuecomment-1809397848 for context.

Effectively, if the checks added the suppression labels because suppressions were indeed added to TypeSpecs (and swaggers?), the hope is that the REST API reviewers et. al. can help them remove the need; thus, the checks should rescan source for suppressions and, if no new suppressions are found compared to the baseline, the suppression label(s) should be removed to avoid blocking the PR like Azure/azure-rest-api-specs#25817 was blocked from past suppressions that were removed.