fix regression: fix internal MNCC operation

While developing the inter-MSC handover refactoring, I was annoyed by the fact
that mncc_tx_to_cc() receives an MNCC message struct containing a msg_type, as
well as a separate msg_type argument, which may deviate from each other. So, as
a first step I wanted to make sure that all callers send identical values for
both by inserting an OSMO_ASSERT(msg_type == msg->msg_type). Later I was going
to remove the separate msg_type argument.

I then forgot to
- carry on to remove the argument and
- to actually test with internal MNCC (it so happens that all of our ttcn3
  tests also use external MNCC).

As a result, the "large refactoring" patch for inter-MSC Handover breaks
internal MNCC operation.

Fix that: remove the separate msg_type argument and make sure that all callers
of mncc_tx_to_cc() indeed pass the desired msg_type in msg->msg_type, and hence
also remove the odd duality of arguments.

Various functions in mncc_builtin.c also exhibit this separate msg_type
argument, which are all unused and make absolutely no sense. Remove those as
well.

Related: OS#3989
Change-Id: I966ce764796982709ea3312e76988a95257acb8d
diff --git a/src/libmsc/gsm_04_08_cc.c b/src/libmsc/gsm_04_08_cc.c
index 0624d56..2869bba 100644
--- a/src/libmsc/gsm_04_08_cc.c
+++ b/src/libmsc/gsm_04_08_cc.c
@@ -2010,11 +2010,10 @@
 }
 
 /* Demux incoming genuine calls to GSM CC from MNCC forwarding for inter-MSC handover */
-int mncc_tx_to_cc(struct gsm_network *net, int msg_type, void *arg)
+int mncc_tx_to_cc(struct gsm_network *net, void *arg)
 {
 	const union mncc_msg *msg = arg;
 	struct mncc_call *mncc_call = NULL;
-	OSMO_ASSERT(msg_type == msg->msg_type);
 
 	if (msg->msg_type == MNCC_SETUP_REQ) {
 		/* Incoming call to forward for inter-MSC Handover? */