Avoid duplicating $response

We need to get it just once, see
https://github.com/phpmyadmin/phpmyadmin/issues/13154#issuecomment-300301726

Signed-off-by: Michal Čihař <michal@cihar.com>

user_password.php

@@ -89,7 +89,6 @@ function PMA_getChangePassMessage($change_password_message, $sql_query = '')
         /**
          * If in an Ajax request, we don't need to show the rest of the page
          */
-        $response = Response::getInstance();
         if ($change_password_message['error']) {
             $response->addJSON('message', $change_password_message['msg']);
             $response->setRequestStatus(false);