libceph: kill ceph_none_authorizer::reply_buf

We never receive authorizer replies with cephx disabled, so it is
bogus.  Also, it still uses the old zero-length array style.

Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>