Cross-repository pull requests prevent deletion of personal forks in some cases

As a Stash/Bitbucket Server User I want to be able to delete my personal fork even though I have a pull request (open OR declined) to an upstream repo to which I no longer have permission to access.

HTR

1. Fork a repo to your personal project.
2. Open a PR from your fork to the upstream repo.
3. Have a user with Project Admin or higher perms remove your permission to the upstream repo.
4. Push another change to your personal fork that would update the PR (the PR shouldn't be visible to you anymore at this point).
5. Delete your fork.

Expected Result

My forked repo is deleted successfully.

Actual Result

Fork deletion fails and an error modal is displayed:

Repository deletion was canceled.
Pull requests involving ~DROHAN/blues could not be cleaned up. The repository may not be delete