Get rid of webpagetest.

Summary:
We're using graphite now to get data from *all* our clients -- much
better data than the 100-200 samples a day we could get from hosted
webpagetest.  So let's get rid of webpagetest.  I feel like all these
graphs are going away anyway; this is just a start.

Test Plan:
Fingers crossed.  I did do
   grep webpagetest -r *
from the analytics homedir and it now gives no results.

Reviewers: jace, chris

Reviewed By: chris

Subscribers: mattfaus

Differential Revision: http://phabricator.khanacademy.org/D8608