Remove some ios-specific oauth logic from khan-exercises. This is being moved to a JS file only added to the ios app.

Summary: Requires sister diff for full functionality/test plan. Comin' soon.

Test Plan: Coming soon in sister diff, will add note here

Reviewers: laura, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D8828