Performance empty pull request rescope activities is very poor on MySQL

STASH-4517 introduced a recurring background job that deletes empty rescope activities. Unfortunately, this delete query performs very poorly on MySQL when there are lots of records in the sta_activity_* tables. This can result in both an increase in deadlocks and extremely long delays on other SQL operations.

Another action which can trigger these slow operations is deleting a repository, since that requires deleting all of the repository's pull requests (and their activities).