Make the lack of authentication explicit on CreateAgent

Part of #1540 (closed)

This is purely for documentation purposes. CreateAgent does not care at all about the authentication, so we just remove all authentication classes, meaning the authentication is not checked at all. Sure, send all the Authorization you want, we don't care.

No updates necessary on the unit tests, since they already test that you do not need any authentication to register as a new agent (the actual authentication being done by the farm's seed and the key exchange stuff).