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/qemu-nbd.c b/qemu-nbd.c
index 6aaebe7..e76fe30 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -354,7 +354,7 @@
 
     nb_fds++;
     nbd_update_server_watch();
-    nbd_client_new(NULL, cioc, tlscreds, NULL, nbd_client_closed);
+    nbd_client_new(cioc, tlscreds, NULL, nbd_client_closed);
 }
 
 static void nbd_update_server_watch(void)