[willow] Legacy Libraries DEPR: Remove re-use support #38058
Description
As early as May 1, 2026, and after Verawood is cut:
- Remove the big add Legacy Library button from courses, removing the ability to create new legacy randomized content components (LegacyLibraryContentBlock)
- Note: Existing usages should still work, and should be migratable if the source library is migrated.
- Note: The randomized settings editor for this component should still work.
- Note: Hitting "View all" should still bring you to the MFE-based children view
- Remove legacy libraries logic from the children view if it's still there.