libmsc/gsm_09_11.c: clean up the local GSM 04.80 API

Since we don't process SS/USSD requests in OsmoMSC anymore, there
are some useless GSM 04.80 functions remained from the past.

In particular, this change does the following:

  - removes both gsm0480_send_{ussd_response|return_error}
    functions because they are not used anymore;

  - changes symbol prefix from 'gsm0480_' to 'msc_', in order to
    avoid possible conflicts with the libosmogsm's GSM 04.80 API;

  - cleans up useless includes;

Change-Id: I2990d8627bce0ce6afb1dcf6b11bb194292380d3
diff --git a/include/osmocom/msc/gsm_04_80.h b/include/osmocom/msc/gsm_04_80.h
index 7d63088..c448c91 100644
--- a/include/osmocom/msc/gsm_04_80.h
+++ b/include/osmocom/msc/gsm_04_80.h
@@ -1,18 +1,10 @@
 #pragma once
 
-#include <osmocom/core/msgb.h>
-#include <osmocom/gsm/protocol/gsm_04_80.h>
-#include <osmocom/gsm/gsm0480.h>
+#include <stdint.h>
 
 struct gsm_subscriber_connection;
 
-int gsm0480_send_ussd_response(struct gsm_subscriber_connection *conn,
-			       uint8_t transaction_id, uint8_t invoke_id,
-			       const char *response_text);
-int gsm0480_send_ussd_return_error(struct gsm_subscriber_connection *conn,
-				   uint8_t transaction_id, uint8_t invoke_id,
-				   uint8_t error_code);
-int gsm0480_send_ussd_reject(struct gsm_subscriber_connection *conn,
+int msc_send_ussd_reject(struct gsm_subscriber_connection *conn,
 			     uint8_t transaction_id, int invoke_id,
 			     uint8_t problem_tag, uint8_t problem_code);