[JBIDE-13761] removed IUser#getAuthKey and IUser#getAuthIV

In UserResource/IUser there are still #getAuthIV #getAuthKey methods that we inherited from the legacy client. They're currently not implement and return <code>null</code>. In the new client the auth keys are passed over to the http client via connection builder. They're not handled by the user any more. We should simply get rid of them.