There is no need for the `Manager.DeleteVolumeGroup()` function as
`VolumeGroup.Delete()` should cover everything too.
By moving the `.Delete()` functionality of removing the group from the
journal to the shared `commonVolumeGroup` type, a volume group snaphot
can use it as well.
Signed-off-by: Niels de Vos <ndevos@ibm.com>