remove dead code after split from openbsc.git
osmo-sgsn.git works as is, but it may still have bits of dead code that came along from openbsc.git.
e.g. the gsup_client and oap_client are duplicated between osmo-msc and osmo-sgsn.
Review all code, remove unused struct members, drop unused functions, eliminate code dup where it makes sense.
Also verify the build, dependencies, installed files.