minor #5711 removed service call from controller (sloba88)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #5711).

Discussion
----------

removed service call from controller

| Q             | A
| ------------- | ---
| Doc fix?      | [yes]
| New docs?     | [no]
| Applies to    | [2.6, 2.7, 2.8]
| Fixed tickets | []

As said in #5696 removed left-over unnecessary service call from controller

Commits
-------

42fa416 removed service call from controller

cookbook/security/voters.rst

@@ -224,8 +224,6 @@ from the authorization checker is called.
             // get a Post instance
             $post = ...;
 
-            $authChecker = $this->get('security.authorization_checker');
-
             $this->denyAccessUnlessGranted('view', $post, 'Unauthorized access!');
 
             return new Response('<h1>'.$post->getName().'</h1>');