code: def delete_thing_shadow(self, thing_name):
          
          thing = iot_backends[self.region_name].describe_thing(thing_name)
          if thing.thing_shadow is None:
              raise ResourceNotFoundException()
          payload = None
          new_shadow = FakeShadow.create_from_previous_version(thing.thing_shadow, payload)
          thing.thing_shadow = new_shadow
          return thing.thing_shadow
docs: after deleting, get_thing_shadow will raise ResourceNotFound.
          But version of the shadow keep increasing...