Fix compiling with LWIP_NOASSERT (and debug/error disabled, too)
diff --git a/src/api/sockets.c b/src/api/sockets.c
index d49e641..24b3b5f 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -1214,9 +1214,7 @@
   struct netconn *conn;
   int i;
 
-#if !LWIP_IPV6
   LWIP_UNUSED_ARG(domain); /* @todo: check this */
-#endif /* LWIP_IPV6 */
 
   /* create a netconn */
   switch (type) {