Recently people have started using the posse tag. However the clean-up tag has been around much longer and serves mostly the same purpose. Half of the 12 questions with posse are already tagged with clean-up, and most of the others have the text "clean up" in their title or body.
I don't deny that posse is probably cooler. Its origins are from Programmers' efforts to gather a posse to help clean up a humongous fraction of their site.
Here, though, a lot of our cleanup jobs don't need a whole posse. Some are just small jobs that a single individual could do, while others are about the etiquette of cleanup jobs. Really, the combined tag operates a lot like retag-request.
I've thus gone and merged them into clean-up. The final name can be changed at any point, but I felt that the consolidation was important and we had no reason to continue with them separate.