mgcp_trunk: remove audio_name and audio_payload

get rid of deprecated trunk parameters which seem to be leftovers
from the old osmo-bsc_mgcp implementation. This is in particular
audio_name and audio_payload in struct mgcp_trunk_config which
allowed the user to "hardcode" an andio name and payload type
via VTY configuration

The removal of the struct members above also require a change to
mgcp_codec.c. The code that is is never actively used and even
causes wrong behavior when activated (set the no-transcoding
flag in VTY). Since the code is removed also the unit tests
also require to be changed to match the new behavior.

Change-Id: Ia050ec3cd34b410dfe089c41b977ae3d5aed7354
Related: OS#2659
diff --git a/tests/mgcp/mgcp_test.c b/tests/mgcp/mgcp_test.c
index ed0fda0..d0da18b 100644
--- a/tests/mgcp/mgcp_test.c
+++ b/tests/mgcp/mgcp_test.c
@@ -1458,11 +1458,9 @@
 	OSMO_ASSERT(conn);
 	OSMO_ASSERT(conn->end.codec->payload_type == 18);
 
-	/* Allocate 5@mgw at select GSM.. */
+	/* Allocate 5@mgw and let osmo-mgw pick a codec from the list */
 	last_endpoint = -1;
 	inp = create_msg(CRCX_MULT_GSM_EXACT, NULL);
-	talloc_free(cfg->virt_trunk->audio_name);
-	cfg->virt_trunk->audio_name = "GSM/8000";
 	cfg->virt_trunk->no_audio_transcoding = 1;
 	resp = mgcp_handle_message(cfg, inp);
 	OSMO_ASSERT(get_conn_id_from_response(resp->data, conn_id,
@@ -1474,7 +1472,7 @@
 	endp = cfg->virt_trunk->endpoints[last_endpoint];
 	conn = mgcp_conn_get_rtp(endp, conn_id);
 	OSMO_ASSERT(conn);
-	OSMO_ASSERT(conn->end.codec->payload_type == 3);
+	OSMO_ASSERT(conn->end.codec->payload_type == 0);
 
 	inp = create_msg(MDCX_NAT_DUMMY, conn_id);
 	last_endpoint = -1;