nbd/server: drop old-style negotiation

After the previous commit, nbd_client_new's first parameter is always
NULL. Let's drop it with all corresponding old-style negotiation code
path which is unreachable now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181003170228.95973-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: re-wrap short line]
Signed-off-by: Eric Blake <eblake@redhat.com>
diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index 1ef1104..1d170c8 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -36,8 +36,7 @@
                        gpointer opaque)
 {
     qio_channel_set_name(QIO_CHANNEL(cioc), "nbd-server");
-    nbd_client_new(NULL, cioc,
-                   nbd_server->tlscreds, NULL,
+    nbd_client_new(cioc, nbd_server->tlscreds, NULL,
                    nbd_blockdev_client_closed);
 }