[DDC-3179] EntityNotFoundException on the postRemove event if the entity is a proxy

This PR tries to fix [DDC-3179](http://www.doctrine-project.org/jira/browse/DDC-3179).

I'm not sure if it is the way to go as this PR has a big drawback: it **always** loads proxies which needs to be removed regardless if there is an postRemove event for it. Anyway, there is no way to determine if there is an event for a specific entity in a lifecycle callback context.

For now, to fix it in our side, we have registered a listener on the preRemove event and load the proxy there.
Our use case it's to remove files which are stored in our entity (we rely on VichUploaderBundle). Then, the bundle attaches a postRemove listener in order to delete them, just after the entity has been removed, not before in case there is something wrong which appends during the remove I suppose.