Changes

rtp_stream: drop use of codecs[] in struct mgcp_conn_peer

struct mgcp_conn_peer in libosmo-mgcp-client API used to have a
mandatory codecs[] array for codec specification and an optional
ptmap[] array to be used only when the default payload type needs
to be overridden.  But now the codecs[] array is deprecated;
the new way is to use ptmap[] to communicate both codecs and
payload types.

These .codecs[] and .codecs_len members in struct mgcp_conn_peer
have been marked as deprecated (and the new way of using only
ptmap[] became available as API) several release cycles ago,
but osmo-msc kept filling out both arrays, leading to compilation
warnings.  Remove this use of deprecated API.

Change-Id: I165be5c00bad3d293db2469ee4bedcbe968afcdc