Stop passing up old stack information to the server.

Summary:
D7679 removed this dependency from the server. It's been
a while since this data was actually filled in with real values,
anyways, so just stop sending them up.

Test Plan:
do some problems in a practice task
do some problems in a mastery task
make sure they all 200

Reviewers: jack, alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D7718