end metacognitive experiment

Summary: khan-exercises change to end current metacognitive experiment.  The corresponding webapp review is http://phabricator.khanacademy.org/D6602 .

Test Plan: Manually test target exercises still load in browser, and that timeline still works for them.

Reviewers: eliana, jace

Reviewed By: eliana

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6601