[stubsabot] Make sure old branches are deleted

[donbarbos]

Sometimes, after a PR is closed, maintainers might forget to remove the branch. I think this could be automated to keep the repository cleaner. At the moment there are 19 branches (-4 not stubsabot), but only a few (8) open stubsabot PRs, which shows that this issue is still relevant.

[srittau]

I honestly don't think this is much of a problem. At least not one that requires significant extra code and extra permissions. The stubsabot PRs are hidden away in their own namespace, so they can easily be ignored.