Remove redundant api client in volume tests

In class method 'setup_clients' of volume tests, it defined api client
'admin_volume_types_client' for twice. This patch is to remove the
redundant one.

Change-Id: Ia91bc8829f2692ab57bc5e2b4d069eff0c94da64
diff --git a/tempest/api/volume/base.py b/tempest/api/volume/base.py
index 8feaca9..c877969 100644
--- a/tempest/api/volume/base.py
+++ b/tempest/api/volume/base.py
@@ -269,7 +269,6 @@
         cls.admin_scheduler_stats_client = \
             cls.os_admin.volume_scheduler_stats_v2_client
         cls.admin_messages_client = cls.os_admin.volume_v3_messages_client
-        cls.admin_volume_types_client = cls.os_admin.volume_types_v2_client
 
     @classmethod
     def resource_setup(cls):