Removing entities with entity manager means that every single notifications entity will be managed
```php 
foreach($user->getNotificationSettings() as $setting) {
  $this->em->remove($setting);
}
```
by calling this method but we expect to deal with entity manager only in the ```update``` and ```delete```  methods to synchronize objects.

As the purpose of this class is to manage the User entity, I think that only this entity should be managed otherwise it could lead to some confusion.

With this way of deleting relationships the synchronisation with DB will be done once we decide to call
```php
$this->em->persist($user);
``` 
and this will automatically delete all user settings from database as well.