Allow clawing back @bors delegate+

[workingjubilee]

I found out that homu doesn't implement the logical @bors delegate- functionality. This is a problem if a reviewer is the sort of person that is often subject to l'esprit de l'escalier.

I know this bot doesn't implement approval-related mechanics yet, but I figured it would make more sense to raise this issue here than the repo which should probably not implement any new functionality.

[workingjubilee]

note that homu seems to document it can do this but doesn't acknowledge it when you actually give it the command.