Update test_live_migration for return-one-value

The _volume_clean_up function missed the change for the return-one-value update
of the volumes client.

Partially implements: blueprint clients-return-one-value

Change-Id: Ie289df1bfebbf27b3f776ff8aa8a5dd5bb37825f
diff --git a/tempest/api/compute/test_live_block_migration.py b/tempest/api/compute/test_live_block_migration.py
index 180dffd..eb71d6d 100644
--- a/tempest/api/compute/test_live_block_migration.py
+++ b/tempest/api/compute/test_live_block_migration.py
@@ -77,7 +77,7 @@
             return server_id
 
     def _volume_clean_up(self, server_id, volume_id):
-        resp, body = self.volumes_client.get_volume(volume_id)
+        body = self.volumes_client.get_volume(volume_id)
         if body['status'] == 'in-use':
             self.servers_client.detach_volume(server_id, volume_id)
             self.volumes_client.wait_for_volume_status(volume_id, 'available')