Remove unused code for updating 'exercise message's to tell user about their current proficiency situation in an exercise. _If_ this code is restored in the future, it will no longer belong in khan-exercise.js

Reviewers: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2048
Merge branch 'master' of http://github.com/Khan/khan-exercises