Changes

Deprecate iu_client

The only remaining use of iu_client is osmo-sgsn.
iu_client was originally meant to be used by osmo-msc
and osmo-sgsn. But the abstraction didn't hold up and
osmo-msc stopped using iu_client for a long time.

To start development, osmo-sgsn will start with a copy of
iu_client until it can be completely dropped from the osmo-sgsn codebase.
Future osmo-sgsn versions won't use the osmo-iuh' iu_client.

Related: OS#5487
Change-Id: I95a5aeccc314ffdeaa04cc93f4e5947f715dd5de