minor #14789 Remove unused imports and properties in experimental authenticator example (jacekpl)

This PR was submitted for the 5.x branch but it was merged into the 5.2 branch instead.

Discussion
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Remove unused imports and properties in experimental authenticator example

Custom [authenticator example](https://symfony.com/doc/current/security/experimental_authenticators.html#creating-a-custom-authenticator) for new experimental authenticator docs contains unused imports and properties. I've simplified it by removing unneded code :-)

Commits
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10f7522 Remove unused imports and properties

Author: OskarStark

security/experimental_authenticators.rst

@@ -287,8 +287,6 @@ method that fits most use-cases::
     // src/Security/ApiKeyAuthenticator.php
     namespace App\Security;
 
-    use App\Entity\User;
-    use Doctrine\ORM\EntityManagerInterface;
     use Symfony\Component\HttpFoundation\JsonResponse;
     use Symfony\Component\HttpFoundation\Request;
     use Symfony\Component\HttpFoundation\Response;
@@ -302,13 +300,6 @@ method that fits most use-cases::
 
     class ApiKeyAuthenticator extends AbstractAuthenticator
     {
-        private $entityManager;
-
-        public function __construct(EntityManagerInterface $entityManager)
-        {
-            $this->entityManager = $entityManager;
-        }
-
         /**
          * Called on every request to decide if this authenticator should be
          * used for the request. Returning `false` will cause this authenticator