There's optional libiu for Iu*S support but it's just a leftover from
pre repo-split times:
* it's not used by any code
* it's not checked by gerrit verification in osmo-ci
* it's disabled by default
* it's not supported in .deb packages
Instead of dragging old code in unknown stage of bit-rotting, let's just
drop it completely.
Change-Id: Id5678a74504b4456aba0438ccc34731770801128