gspn: remove the interface with GreatSPN
It hasn't been tested for several year, may not even compile, has to be linked with source code that isn't even publicly available, and its presence was the only reason to keep some inefficient code in gtec.cc and friends. * iface/gspn/: Delete this directory. * iface/Makefile.am, configure.ac, README: Adjust. * m4/gspnlib.m4: Delete. * src/sanity/Makefile.am: Do not use LIBGSPN_CPPFLAGS.