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/include/osmocom/msc/gsm_04_08.h b/include/osmocom/msc/gsm_04_08.h
index 47747cb..661da42 100644
--- a/include/osmocom/msc/gsm_04_08.h
+++ b/include/osmocom/msc/gsm_04_08.h
@@ -45,7 +45,7 @@
 int gsm48_send_rr_ass_cmd(struct gsm_lchan *dest_lchan, struct gsm_lchan *lchan, uint8_t power_class);
 int gsm48_send_ho_cmd(struct gsm_lchan *old_lchan, struct gsm_lchan *new_lchan, uint8_t power_command, uint8_t ho_ref);
 
-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);
 
 /* convert a ASCII phone number to call-control BCD */
 int encode_bcd_number(uint8_t *bcd_lv, uint8_t max_len,