libmsc/gsm_04_80.c: drop unused 'in_msg' argument

It isn't used, so no need to pass it.

Change-Id: I4248ebf8626d3f7f0bf273fd20cc4f64e979938b
diff --git a/include/osmocom/msc/gsm_04_80.h b/include/osmocom/msc/gsm_04_80.h
index 7da45a0..dedf243 100644
--- a/include/osmocom/msc/gsm_04_80.h
+++ b/include/osmocom/msc/gsm_04_80.h
@@ -7,10 +7,9 @@
 struct gsm_subscriber_connection;
 
 int gsm0480_send_ussd_response(struct gsm_subscriber_connection *conn,
-			       const struct msgb *in_msg, const char* response_text,
+			       const char* response_text,
 			       const struct ss_request *req);
 int gsm0480_send_ussd_reject(struct gsm_subscriber_connection *conn,
-			     const struct msgb *msg,
 			     const struct ss_request *request);
 
 int msc_send_ussd_notify(struct gsm_subscriber_connection *conn, int level,