[nat] Remove parameter that is never accessed directly

The msgb needs to be around when we access the parsed structure
but that needs to be guranteed by the caller handing out the parsed
structure.
diff --git a/openbsc/src/nat/bsc_sccp.c b/openbsc/src/nat/bsc_sccp.c
index f77cbef..47cd9ce 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -82,7 +82,7 @@
 	return -1;
 }
 
-int create_sccp_src_ref(struct bsc_connection *bsc, struct msgb *msg, struct bsc_nat_parsed *parsed)
+int create_sccp_src_ref(struct bsc_connection *bsc, struct bsc_nat_parsed *parsed)
 {
 	struct sccp_connections *conn;