minor #3929 Update custom_authentication_provider.rst (verschoof)

This PR was submitted for the 2.5 branch but it was merged into the 2.4 branch instead (closes #3929).

Discussion
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Update custom_authentication_provider.rst

Removed response because it will always be overwritten "By default deny authorization" response

Commits
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d136ac8 Update custom_authentication_provider.rst

cookbook/security/custom_authentication_provider.rst

@@ -159,12 +159,6 @@ set an authenticated token in the security context if successful.
                 //     $this->securityContext->setToken(null);
                 // }
                 // return;
-
-                // Deny authentication with a '403 Forbidden' HTTP response
-                $response = new Response();
-                $response->setStatusCode(Response::HTTP_FORBIDDEN);
-                $event->setResponse($response);
-
             }
 
             // By default deny authorization