Remove unused func _detach

Func _detach is not used in test_volumes_snapshots, so this is
to remove it.
Besides, func cleanup_snapshot should be placed before testcases.

Change-Id: I961a3400a3777c6032815e2ee9c68f0cd76a43c2
diff --git a/tempest/api/volume/test_volumes_snapshots.py b/tempest/api/volume/test_volumes_snapshots.py
index 6be569c..f971eca 100644
--- a/tempest/api/volume/test_volumes_snapshots.py
+++ b/tempest/api/volume/test_volumes_snapshots.py
@@ -34,11 +34,11 @@
         cls.name_field = cls.special_fields['name_field']
         cls.descrip_field = cls.special_fields['descrip_field']
 
-    def _detach(self, volume_id):
-        """Detach volume."""
-        self.volumes_client.detach_volume(volume_id)
-        waiters.wait_for_volume_status(self.volumes_client,
-                                       volume_id, 'available')
+    def cleanup_snapshot(self, snapshot):
+        # Delete the snapshot
+        self.snapshots_client.delete_snapshot(snapshot['id'])
+        self.snapshots_client.wait_for_resource_deletion(snapshot['id'])
+        self.snapshots.remove(snapshot)
 
     @test.idempotent_id('b467b54c-07a4-446d-a1cf-651dedcc3ff1')
     @test.services('compute')
@@ -121,12 +121,6 @@
         self.assertEqual(volume['snapshot_id'], src_snap['id'])
         self.assertEqual(int(volume['size']), src_size + 1)
 
-    def cleanup_snapshot(self, snapshot):
-        # Delete the snapshot
-        self.snapshots_client.delete_snapshot(snapshot['id'])
-        self.snapshots_client.wait_for_resource_deletion(snapshot['id'])
-        self.snapshots.remove(snapshot)
-
 
 class VolumesV1SnapshotTestJSON(VolumesV2SnapshotTestJSON):
     _api_version = 1