Fix AccountDeletionWorker crashing and clogging sidekiq queues (#15380)

* Fix account deletion workers being queued multiple times for a single account

* Fix poll votes being unnecessarily instantiated on poll deletion

* Fix favourites being unnecessarily instantiated on status deletion

* Remove inaccurate comments

* Delete polls instead of destroying them

Co-authored-by: Claire <claire.github-309c@sitedethib.com>