Girder shutdown

[jwnimmer-tri]

It's time to shut down https://drake-girder.csail.mit.edu/. We aren't using it anymore, so we should save the money (and maintenance cost).

Let's do it in phases:

(1) When next we have some free time, we should turn off the infrastructure -- but not delete anything.

(2) Then maybe 3 months later, we'll actually go and delete (or glacier?) the S3 bucket, delete all of the server images and/or configs, whatever makes sense. Email MIT to disable the CNAME, etc.