cinder: Remove redundant command

There is no 'volume group unset' command nor any need for one right now.
This was mistakenly added in I3b2c0cb92b8a53cc1c0cefa3313b80f59c9e5835.

Change-Id: I9386d1350099b10659c6b0e632e4d83cae5b2bfd
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

Author: stephenfin

releasenotes/notes/add-volume-group-commands-b121d6ec7da9779a.yaml

@@ -3,6 +3,6 @@ features:
   - |
     Add ``volume group create``, ``volume group delete``,
     ``volume group list``, ``volume group failover``,
-    ``volume group set/unset`` and ``volume attachment show``
+    ``volume group set`` and ``volume attachment show``
     commands to create, delete, list, failover, update and show volume groups,
     respectively.

setup.cfg

@@ -726,7 +726,6 @@ openstack.volume.v3 =
     volume_group_list = openstackclient.volume.v3.volume_group:ListVolumeGroup
     volume_group_failover = openstackclient.volume.v3.volume_group:FailoverVolumeGroup
     volume_group_set = openstackclient.volume.v3.volume_group:SetVolumeGroup
-    volume_group_unset = openstackclient.volume.v3.volume_group:UnsetVolumeGroup
     volume_group_show = openstackclient.volume.v3.volume_group:ShowVolumeGroup
 
     volume_group_snapshot_create = openstackclient.volume.v3.volume_group_snapshot:CreateVolumeGroupSnapshot