Remove unnecessary persist call

From [the documentation](http://symfony.com/doc/current/book/doctrine.html#updating-an-object):
> Notice that calling $em->persist($product) isn't necessary. Recall that this method simply tells Doctrine to manage or "watch" the $product object. In this case, since you fetched the $product object from Doctrine, it's already managed.

Would there be any other reasons to do this call to `persist()` ?