Remove stalebot

[rnorth]

Because we should.

I'd like to do two things in the near future:

• discuss our triage process; essentially if we're as robust about triaging as we have been about marking things as stale, we'd be in a better place. A big part of making sure we triage things well is probably just reminding myself to do it.
• do an efficient skim of all issues and PRs that were closed as stale. I want to believe that important items will have been kept open, but I'm sure we'll find exceptions that we can learn from.