Remove redundant volume check in nova_volume_detach

In nova_volume_detach, show_volume and assertEqual are
redundant because in wait_for_volume_resource_status
this check is already done. This is to remove them for
code clean.

Change-Id: If5a25914bd4be039e0a8ddad661732ef1f3a28ee
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 2843222..ef2dd3a 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -506,9 +506,6 @@
         waiters.wait_for_volume_resource_status(self.volumes_client,
                                                 volume['id'], 'available')
 
-        volume = self.volumes_client.show_volume(volume['id'])['volume']
-        self.assertEqual('available', volume['status'])
-
     def rebuild_server(self, server_id, image=None,
                        preserve_ephemeral=False, wait=True,
                        rebuild_kwargs=None):