Auto merge of #6463 - dwijnand:rip-stale-bot, r=alexcrichton

Remove Stale bot's configuration

We couldn't get stale bot to operate as we'd like (poke up N
older-then-X-period tickets a day) and therefore we're tearing it down.

We've chosen to keep the "stale" label, so tickets closed by the bot
may be found with that label.

Closes #6035