drop (now) unused code

Removing the duality of codecs[] and ptmap[] in structs mgcp_msg,
mgcp_response and mgcp_conn_peer has removed the need to "map" from
codec type enum to payload type number. They are stored together now.

Remove functions that are no longer used.
None of our osmocom users of libosmo-mgcp-client call these functions.

Change-Id: I84e5285831397c992af59deee12dea8458d16cc6
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 964ffe1..f1a702f 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -38,3 +38,8 @@
 libosmo-mgcp-client	deprecate public API	New code should no longer use codecs[], instead use ptmap[].codec. There
 						is backwards compat code that moves codecs[] entries, if any, over to
 						ptmap[], so callers may migrate at own leisure.
+libosmo-mgcp-client	remove public API	Since codecs[] has been deprecated in favor of ptmap[], there is no use
+						for the following functions; all known callers have been within
+						osmo-mgw.git:
+						map_codec_to_pt()
+						map_pt_to_codec()