Changes

Drop iu_client - adopted by osmo-sgsn

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.
Move iu_client to osmo-sgsn until it got replaced by a more
direct implementation (similar to IUCS in osmo-msc).

Related: OS#5487
Change-Id: I95a5aeccc314ffdeaa04cc93f4e5947f715dd5de