Clean up after track selection experiment

This cleans up the code left over from the track selection experiment ([ECOM-16](https://openedx.atlassian.net/browse/ECOM-16)). These changes don't include cleanup of the SCSS used to style the `choose`, `_verification_header`, and `_verification_support` templates; that seemed like a separate story, and UX is out today.

@wedaly and @stephensanchez, could you review this?
Step 4 is only possible if the user navigates directly to the track selection page by pasting the URL for the course they were previously enrolled in into their browser. The majority of users in the situation you describe will go back to the course about page and click the iframe, which automatically sets their enrollment in the course back to active before routing them to the track selection page. As my comment describes, this is now the only way to enroll in a course's honor track - we don't want people to end up at the track selection page unable to enroll in the honor track.