Remove unnecessary tearDown function in testcase

In testcase, tearDown will be called automatically. This patch used to
remove tearDown functions that do nothing. Besides, it will keep code clean

Change-Id: I3a8977fb1ea035acd0da71233e184ed293619556

nova/tests/unit/virt/libvirt/storage/test_rbd.py

@@ -86,9 +86,6 @@ class RbdTestCase(test.NoDBTestCase):
         self.volume_name = u'volume-00000001'
         self.snap_name = u'test-snap'
 
-    def tearDown(self):
-        super(RbdTestCase, self).tearDown()
-
     @mock.patch.object(rbd_utils, 'rbd')
     def test_rbdproxy_wraps_rbd(self, mock_rbd):
         proxy = rbd_utils.RbdProxy()

nova/tests/unit/virt/libvirt/test_driver.py

@@ -17730,9 +17730,6 @@ class LibvirtVolumeSnapshotTestCase(test.NoDBTestCase):
                                          'merge_target_file':
                                              'some_other_file'}
 
-    def tearDown(self):
-        super(LibvirtVolumeSnapshotTestCase, self).tearDown()
-
     @mock.patch('nova.virt.block_device.DriverVolumeBlockDevice.'
                 'refresh_connection_info')
     @mock.patch('nova.objects.block_device.BlockDeviceMapping.'