remove AuthPlugin usage from CORE SignupApi extension

* 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::allowPasswordChange() always returned true.
* AuthPlugin::initUser() has no logic whatsoever.
* AuthPlugin::modifyUITemplate() does not change anything.