Removed the Mergeable check

Now that we are syncing all PRs from Piper with Copybara, there isn't a convenient workflow for adding per-language labels to the GitHub PR.  (Also even if we required this, Copybara sometimes closes the PR and makes a separate commit, instead of merging the PR).

Long-term we want the release script to bucket by language by automatically detecting which files were modified.  In the near-term, the onduty will need to do this manually at release time.

PiperOrigin-RevId: 485730509