doc: cgroups: Remove unwanted references to systemd
doc: cgroups: Remove unwanted references to systemd

The non-systemd configurations do not create system neither user
control groups.  The title of the diagram referenced systemd too.

Signed-off-by: Miguel Ángel Arruga Vivas <8bd72548c2500120f567090f725dc762e6857e9c@gmail.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jardasgit/libvirt,libvirt/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,fabianfreyer/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt