Update update.class.php

Removed setting $this->classKey to avoid error when trying to change the class_key of a user. The inheritance to modUser of a custom user class will make it load modUser record anyway
[#9973] Prevent extended user classes being set to modUser

Merge remote-tracking branch 'origin/pr/616' into release-2.2

* origin/pr/616:
  Update update.class.php