[willow] Legacy Libraries DEPR: Remove create/edit/view/config support

[kdmccormick]

This can be done after the Verawood cutoff, as early as May 1, 2026.

Steps:

• Remove the legacy libraries creation page (i.e., revert this: feat: Add ability to create Legacy Libraries frontend-app-authoring#2551)
• Remove the legacy library viewing + editing page
• Remove the legacy library user access page
• Remove the legacy library import page
• Remove the legacy library export page
• Search and destroy unused REST APIs and utility code