MissedMessageWorker: Remove unnecessary transaction.atomic.

We only have one query which will change database state in this function,
and we already have a lock on the process itself, so there's no need for
a transaction.

This was added in ebb4eab0f9.

zerver/worker/queue_processors.py

@@ -587,7 +587,6 @@ class MissedMessageWorker(QueueProcessingWorker):
             user_profile_id: int = event["user_profile_id"]
             batch_duration = datetime.timedelta(seconds=self.BATCH_DURATION)
 
-            with transaction.atomic():
             try:
                 pending_email = ScheduledMessageNotificationEmail.objects.filter(
                     user_profile_id=user_profile_id