Remove the old Review Bot runtests.py test runner.
Remove the old Review Bot runtests.py test runner.

Review Bot uses pytest these days, but we still had the old test runner
around, which used nose. This isn't compatible with the modern codebase,
and was confusing to have here.

This is a trivial change that just deletes the old test runner.

reviewboard/ReviewBot,reviewboard/ReviewBot,reviewboard/ReviewBot,reviewboard/ReviewBot