Remove Proxy from EntityManagerInterface contract

[derrabus]

After merging #8922 by @simPod, I had a deeper look that the implementation to see if we can fix some of the newly baselined errors. I'd like to partially revert the change now. Sorry for the confusion.

#8922 added an intersection with the Proxy interface to the return type of EntityManagerInterface::getReference() and getPartialReference(). Here's why I now think that this is problematic:

1. While our implementation EntityManager might return such an object, we cannot guarantee that it will. If you look at the following code snippet, you can see that the actual entity is returned instead of a reference, if the entity has already been registered with the UoW. In that case, the returned entity might not be a proxy object.

   orm/lib/Doctrine/ORM/EntityManager.php

   Lines 515 to 520 in 04d28a9

   	$entity = $this->unitOfWork->tryGetById($sortedId, $class->rootEntityName);
   	// Check identity map first, if its already in there just return it.
   	if ($entity !== false) {
   	return $entity instanceof $class->name ? $entity : null;
   	}

   If we kept the Proxy interface in the return type, the correct type would be (T&Proxy)|T|null.

2. The Proxy interface is somewhat an implementation detail of our EntityManager implementation that should not be leaked to the interface. I believe, the ORM should be able to move to a different proxy library without violating the contract of EntityManagerInterface.

[greg0ire]

Thanks @derrabus !