Get rid of stuff we haven't used for a long time.

Summary:
The source mirrors: gone.  (Phabricator does that automatically for us
now.)

python-hipchat: gone.  (We use alertlib now.)

mercurial stuff: gone.  (We use only git now.)

Test Plan: Fingers crossed.

Reviewers: chris, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D14697