packages: restore chkconfig.save

install_packages_disabled() now properly restore chkconfig from
the backup
remove_system_on_rhn: minor cleanup
remove_system_on_rhn: remove the duplicated record

remove every RHN record with the same hostname.
repository: fix remove_system_on_rhn path
repositories: avoid the dependency on server-sfs-6

rhel-x86_64-server-sfs-6 is an optional dependency. Let's enable
it only if needed.