Remove use_sonata_admin from cmf_core.persistence.phpcr config

Removing the use_sonata_admin settings ensures it defaults to auto, which is necessary in order for the sonata admin services to be registered and used by the cmf if the sonata admin bundle has been registered.

Discussion: https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/168#issuecomment-23747056