Remove dead tests fixtures

Author: fancyweb

Tests/ResponseTest.php

@@ -1001,14 +1001,3 @@ public function __toString()
 class DefaultResponse extends Response
 {
 }
-
-class ExtendedResponse extends Response
-{
-    public function setLastModified(\DateTime $date = null)
-    {
-    }
-
-    public function getDate()
-    {
-    }
-}