Unbind window.message in componentWillUnmount

Summary: If we don't unbind, then we get an error amount not being able to mount.

Test Plan:
- Have multiple items in an exercise, one of which has an iframe
- Complete the first one, click to go to next
- Don't see error
- This can be done live at: https://www.khanacademy.org/binary-search/e/card-finder-with-binary-search/edit .

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11245