* feat!: Removing the long-deprecated legacy course_modes chooser
`course_modes/choose.html` (and its corresponding
`_upgrade_button.html`) were specifically only used for the edge case
where an enterprise user found themselves in the non-enterprise learner
dashboard, and attempted to enroll in a course outside of the
enterprise flow. Once upon a time, in a 2U-only workflow, the commerce
system would apply specific discounts for users within the said case.
That's no longer true, and it has never been true outside of this one
company.
Removing this template cleans up a legacy version of a legacy page that
was, realistically, exclusively seen by employees of 2U,
and nobody else.
Removes:
* The corresponding testsfor behavior only seen in the legacy page.
* A waffle flag since all cases route as if the flag is set: `VALUE_PROP_TRACK_SELECTION_FLAG`: `course_modes.use_new_track_selection`
* Some variables set in `CourseModeView` which were only ever rendered in the legacy template (`title_content`, `has_credit_upsell`) have been removed from the class.
* There is a high likelihood that the class is still a target for re-factoring now that the legacy view is gone, but I'm hesitant to touch something which is not covered by previously existing tests, because the logic around what template gets rendered when is complex.
FIXES: APER-3779
FIXES: #36090
da348fa456