Once upon a time, integration with Slack was done through an extension

that we shipped. This wasn't a great implementation and was quite

limited, and we ended up completely rewriting it for the

rbintegrations, which is now always installed alongside Review Board.

We kept the old rbslack implementation in git, but Review Board has

now included integrations for a long time, and this codebase is no

longer useful. This change gets rid of it.