deb: remove unused variable in debian/rules

python_version is never used, it's just old cruft.
deb: debian/rules doesn't need to care about BUILD_SYS

The -X argument to dh_fixperms says to eXclude linuxcnc_module_helper
when it's fixing the permissions on all our files.  But dh_fixperm won't
complain or fail if it doesn't find linuxcnc_module_helper, so we don't
need to do this substitution trick to tell it what to expect.
deb: dont .gitignore package-dirs we no longer build
deb: get rid of ancient Provides in control

I think it's time to stop worrying about users upgrading from emc2.deb
packages.