Fix EZP-23752: Strict standards error with Symfony 2.6

> https://jira.ez.no/browse/EZP-23752

Removed custom `isGranted()` method from `eZ\Bundle\EzPublishCoreBundle\Controller`.
This must go with a Symfony version bump to `~2.6` in ezpublish-community (see https://github.com/ezsystems/ezpublish-community/pull/210)