remove AuthPlugin usage from SpecialUserlogin

* AuthPlugin::validDomain() always returned true.
* AuthPlugin::canCreateAccounts() always returned false.
* AuthPlugin::userExists() always returned false.
* AuthPlugin::authenticate() always returned false.
* AuthPlugin::addUser() always returned true.
* AuthPlugin::initUser() had no logic.
* AuthPlugin::updateUser() had no logic and always returned true.
* AuthPlugin::autoCreate() always returned false.
* AuthPlugin::allowPasswordChange() always returned true.