Delete .html extension-adding

Summary: Line previously added .html and is no longer necessary (maybe). Should I autoSubmitIssue to make sure this is never used _before_ just deleting it?

Test Plan: Make sure you can get to exercises (tried several)

Reviewers: eater, alpert

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6478