When a user is updating their pull request, we should clear the `merging-blocked` label, if such a label is currently applied.
<rdar://problem/91380962>
Pull request: https://github.com/WebKit/WebKit/pull/251
Committed r292538 (249376@main): <https://commits.webkit.org/249376@main> Reviewed commits have been landed. Closing #251 and removing active labels.