Remove extraneous scoping hack

Summary: Previously, there was a scoping problem where exerciseId might change; but now it's a local variable, so this hack should no longer be necessary

Test Plan: Used console.log to make sure that exerciseId got through unscathed.

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6159