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 SASS 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?
Agree with the logic, but I think the comment could be clearer.  Readers won't necessarily know the mechanics of `change_enrollment`, so it might make more sense to say something like "By the time the user gets here during the normal registration process, they will already have an activated enrollment."