Deleting a pull request doesn't delete its public PR refs

Issue Summary

When a PR is opened, BBS creates a public PR ref for the pull request in `$BITBUCKET_HOME/shared/data/repositories/<repo_id>/refs/pull-requests/<pr_id>`. This PR ref is deleted when a PR is declined, but not when the PR is deleted.

This can cause a build up of stale PR refs in BBS home, which can slow down git commands. 

Steps to Reproduce

1. Create a pull request, see that there is a pull request ref for this PR ID
2. Delete the pull request

Expected Results

Pull request ref should be deleted

Actual Results

The ref still exists in `$BITBUCKET_HOME/shared/data/repositories/<repo_id>/refs/pull-requests/<pr_id>`

Workaround

Manually delete the stale PR ref