Remove unnecessary resource_setup()

Since I82b216844b43f3440d1827e66225f1f97e97dd19 , resource_setup()
became useless, so there is no need to keep it, just remove it for
the code clean.

Change-Id: Ib187ce91f2d3d629389fa0581bd051659d551658
diff --git a/tempest/api/volume/test_volumes_backup.py b/tempest/api/volume/test_volumes_backup.py
index 74078cc..86076b7 100755
--- a/tempest/api/volume/test_volumes_backup.py
+++ b/tempest/api/volume/test_volumes_backup.py
@@ -30,10 +30,6 @@
         if not CONF.volume_feature_enabled.backup:
             raise cls.skipException("Cinder backup feature disabled")
 
-    @classmethod
-    def resource_setup(cls):
-        super(VolumesBackupsV2Test, cls).resource_setup()
-
     @test.idempotent_id('a66eb488-8ee1-47d4-8e9f-575a095728c6')
     def test_volume_backup_create_get_detailed_list_restore_delete(self):
         # Create backup