Don't collect webpagetest data anymore.

Summary:
We're moving from webpagetest on analytics machine to hostedgraphite,
which has much more data for us to look at.

Test Plan:
Fingers crossed -- will have to install the new crontab on the
analytics machine after this is committed.

Reviewers: jace, chris

Reviewed By: chris

Subscribers: mattfaus

Differential Revision: http://phabricator.khanacademy.org/D8609