'Delete a device.
Args:
user_id (str): The ID of the user which owns the device
device_id (str): The ID of the device to delete
Returns:
defer.Deferred'

@defer.inlineCallbacks
def delete_device(self, user_id, device_id):

    (yield self._simple_delete_one(table='devices', keyvalues={'user_id': user_id, 'device_id': device_id}, desc='delete_device'))
    self.device_id_exists_cache.invalidate((user_id, device_id))
