refactor: drop orphaned Studio Maintenance underscore template

The Studio Maintenance dashboard (and its Announcements and Force
Publish tools) was removed in #37432 (feat!: Remove Studio Maintenance
& Announcements).  That removal left one orphaned Underscore template
behind: cms/templates/js/maintenance/force-published-course-response.underscore.

Nothing references it -- no JS, RequireJS build entry, Python view, or
URL loads it -- so it is dead code.  Removing it completes the cleanup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>