[y release] Legacy Libraries DEPR: Total deletion

This can be done after the Xylon cutoff, as early as May 1, 2027.

This remove the ability for operators to migrate legacy libraries to V2 libraries!
Why do we need to do this at all? -> In order to remove MongoDB as a platform dependency and simplify the course delivery subsystem.

Steps:

• Figure out a way to retain legacy "library defaults" so that LegacyLibraryContentBlock children continue to render correctly forever.
• Remove LibraryRoot xblock
• Remove legacy library storage from modulestore
• Remove remaining legacy library APIs
• Remove all parts of the LegacyLibraryContentBlock (V1) (except that which would allow a migration to Problem Bank (V2)? TBD below). Remove the logic for managing LegacyLibraryContentBlock (V1) children, including the content_libraries tasks sync_from_library and duplicate_children
• https://github.com/openedx/openedx-platform/issues/35192
• Grep for references to LibraryLocator and LibraryBlockUsageLocator. Destroy as much of the logic as possible. If possible, actually remove those key classes from OpaqueKeys.
• Generally grep for "libraries" and remove any legacy library logic you an find.

What must continue to work when this is done?

• Existing LegacyLibraryContentBlocks should continue working from a student's perspective, forever. Essentially as dumb containers of randomized blocks.
  • TBD: Do we allow migrations to ProblemBanks forever?

Start by mapping legacy library references across modulestore, LibraryRoot, LegacyLibraryContentBlock, OpaqueKeys, and the content_libraries tasks sync_from_library and duplicate_children. Confirm how existing LegacyLibraryContentBlocks retain their defaults and render as randomized containers before removing migration, storage, and API paths. Done means legacy blocks still work for students while MongoDB and the legacy library logic are no longer platform dependencies.