spec: don't enable cgconfig under systemd

In Fedora 16, we quit enabling cgconfig because systemd set up
default cgroups that were good enough for our use.  But in F17,
when we switched to systemd, we reverted and started up cgconfig
again.  See also the tail of this thread:
https://www.redhat.com/archives/libvir-list/2012-October/msg01657.html

* libvirt.spec.in (with_systemd): Rely on systemd for cgroups.
(cherry picked from commit b61eadf3c6)

libvirt.spec.in

@@ -976,7 +976,6 @@ done
 if [ $1 -eq 1 ] ; then
     # Initial installation
     /bin/systemctl enable libvirtd.service >/dev/null 2>&1 || :
-    /bin/systemctl enable cgconfig.service >/dev/null 2>&1 || :
 fi
 %else
 %if %{with_cgconfig}