ranap_common_cn: remove unused ctx args

ranap_cn_rx_co and ranap_cn_rx_cl both have a void *ctx argument, that
gets passed to:
* a callback function and
* to various decode functions in the same file.

As it is named "ctx", it looks like a talloc context. But the decode
functions don't use ctx at all and so in reality it is private userdata
for the callback. It is used as such by test/hnb-test-rua and in
osmo-msc.

Start cleaning this up by removing the unused ctx args, a future patch
will rename it to priv and yet another patch will fix a compiler warning
in test/hnb-test-rua by properly using it as userdata arg.

Change-Id: I8936197d7ae7ffddbe8ee99d909d74ac5b3ab227
---
M TODO-RELEASE
M include/osmocom/ranap/ranap_common_cn.h
M src/ranap_common_cn.c
3 files changed, 35 insertions(+), 27 deletions(-)