When a post notification history is deleted in
MailPoet\Cron\Workers\SendingQueue\Tasks\Newsletter::preProcessNewsletter
it is also detached from EntityManager.
Any further attempt to manipulate the entity via EntityManager (remove or flush) causes
errors like "ERROR: A new entity was found through the relationship 'MailPoet\Entities\ScheduledTaskEntity#sendingQueue'"
In this commit we prevent doing such changes.
[MAILPOET-5880]