Remove seat data syncing services and make workers no-op (deprecation step M)

What does this MR do?

This is step M of the 3-release Sidekiq worker removal process for GitlabSubscriptions::Members::AddedWorker and DestroyedWorker, per Removing worker classes. They cannot be deleted outright because they are event-driven Sidekiq workers, so an outright removal risks a version mismatch between the process that enqueues an event and the process that handles it.

• Unsubscribe AddedWorker and DestroyedWorker from their events in gitlab_subscriptions_subscriptions.rb, so no new jobs are enqueued.
• Replace handle_event on both workers with a no-op, so any already-queued jobs drain harmlessly.
• Delete AddedService, the synchronous service AddedWorker called; nothing else references it once handle_event no-ops.

RecordLastActivityWorker/ActivityService, the orphaned SeatAssignments::SyncService, and SeatTypeCalculator are left in place.

Subsequent merge requests

• Step M+1 (purge any already-queued job instances): !250048 (merged)
• Step M+2 (delete the worker classes entirely): !250058

Both are marked as draft and stacked on top of this MR, since they depend on this one merging (and, for M+2, on M+1 merging) first.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/606016