Remove unnessessary metadata parameter

The metadata was class value and it was used in some tests.
However, the value has been unused since I7cecbde9543c441a2f3478941f85625755aabf7f

Change-Id: I159663dcb463a3a218358c56291ca97fff9f8202
diff --git a/tempest/api/volume/v2/test_volumes_list.py b/tempest/api/volume/v2/test_volumes_list.py
index 8b51e64..d2328c8 100644
--- a/tempest/api/volume/v2/test_volumes_list.py
+++ b/tempest/api/volume/v2/test_volumes_list.py
@@ -37,13 +37,12 @@
         super(VolumesV2ListTestJSON, cls).resource_setup()
 
         # Create 3 test volumes
-        metadata = {'Type': 'work'}
         # NOTE(zhufl): When using pre-provisioned credentials, the project
         # may have volumes other than those created below.
         existing_volumes = cls.volumes_client.list_volumes()['volumes']
         cls.volume_id_list = [vol['id'] for vol in existing_volumes]
         for _ in range(3):
-            volume = cls.create_volume(metadata=metadata)
+            volume = cls.create_volume()
             cls.volume_id_list.append(volume['id'])
 
     @decorators.idempotent_id('2a7064eb-b9c3-429b-b888-33928fc5edd3')