Fix Doctrine MediaManager to implement MediaManagerInterface too

After adding the `Sonata\CoreBundle\Entity\DoctrineBaseManager` to this `Entity\MediaManager`, the `MediaManagerInterface` was not implemented anymore.

This results in a fatal error on a video view page because Pixlr class is waiting for a `MediaManagerInterface` in the constructor.
*See: https://github.com/sonata-project/SonataMediaBundle/blob/master/Extra/Pixlr.php#L54*

As the save() and delete() methods are already verified by the `Sonata\CoreBundle\Entity\ManagerInterface` I think we can remove these methods from the `MediaManagerInterface`.