Remove `BulkImports::StuckImportWorker` class

!139676 (merged) renamed BulkImports::StuckImportWorker, leaving the old class in place but changing the perform method into a no-op.

Since the previous MR has been merged and deployed to production, app/workers/bulk_imports/stuck_import_worker.rb should now be safe to remove.

https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes