[Security] Remove everything related to the deprecated authentication manager

Author: wouterj

AppVariable.php

@@ -78,10 +78,7 @@ public function getUser(): ?object
             return null;
         }
 
-        $user = $token->getUser();
-
-        // @deprecated since 5.4, $user will always be a UserInterface instance
-        return \is_object($user) ? $user : null;
+        return $token->getUser();
     }
 
     /**

Tests/AppVariableTest.php

@@ -95,13 +95,6 @@ public function testGetUser()
         $this->assertEquals($user, $this->appVariable->getUser());
     }
 
-    public function testGetUserWithUsernameAsTokenUser()
-    {
-        $this->setTokenStorage($user = 'username');
-
-        $this->assertNull($this->appVariable->getUser());
-    }
-
     public function testGetTokenWithNoToken()
     {
         $tokenStorage = $this->createMock(TokenStorageInterface::class);