CVE-2015-5370: s3:rpc_server: just call pipe_auth_generic_bind() in api_pipe_bind_req()

pipe_auth_generic_bind() does all the required checks already
and an explicit DCERPC_AUTH_TYPE_NONE is not supported.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11344

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
