remove obsolete dependency on libosmo-sccp

We only use libosmo-sigtran these days, so we can remove the depdency
to the old libosmo-sccp from osmo-bsc.  We don't use LIBOSMO_SCCP_*
variables in any Makefile.am, nor do we #include <osmocom/sccp/...>
anywhere [anymore].

Change-Id: Ie478016ffb6e767ba10968c1ee2ab98db15a45a3
Related: OS#2601
3 files changed