osmo_msc: remove unused parameter from msc_dtap()

The parameter link_id in the function msc_dtap() is unused. Lets remove
it.

Change-Id: I7ba67b0cb514c91bc87a7b396ed3962b7a68e7da
diff --git a/src/libmsc/a_iface_bssap.c b/src/libmsc/a_iface_bssap.c
index c11347e..9a2333d 100644
--- a/src/libmsc/a_iface_bssap.c
+++ b/src/libmsc/a_iface_bssap.c
@@ -686,7 +686,7 @@
 	OMSC_LINKID_CB(msg) = dtap->link_id;
 
 	/* Forward dtap payload into the msc */
-	msc_dtap(conn, conn->a.conn_id, msg);
+	msc_dtap(conn, msg);
 
 	return 0;
 }