Remove Studio access for Old Mongo courses.

[ormsbee]

All access to Old Mongo courses was removed from the LMS with openedx/openedx-platform#29848

We need to similarly remove all Old Mongo access from Studio. This includes:

• Old Mongo courses should not appear in the listing of courses in the Studio course listing page.
• Any attempt to directly access an Old Mongo course in Studio via URL should also fail with a 404 error.
• The get_courses and get_course_summaries calls in MongoModuleStore should be modified to always return empty lists.

This will likely break a number of tests, which will need to be deleted or modified (if they're still useful).