move header text addition to problem-template.handlebar

Summary: This is the companion commit to http://phabricator.khanacademy.org/D8419 which makes header text appear for both Perseus and old style exercises.  It removes the code from khan-exercise.js which added psych. intervention header text, since headers are now added in problem-template.handlebar instead.

Test Plan: ran on local dev_appserver

Reviewers: colinfuller, mattfaus, alpert

Reviewed By: alpert

Subscribers: benjaminhaley, jace

Differential Revision: http://phabricator.khanacademy.org/D8501