code: def DeregisterMountPoint(cls, mount_point):
      
      if mount_point not in cls._mount_points:
        raise KeyError('Mount point: {0:s} not set.'.format(mount_point))
  
      del cls._mount_points[mount_point]
docs: Deregisters a path specification mount point.
  
      Args:
        mount_point (str): mount point identifier.
  
      Raises:
        KeyError: if the corresponding mount point is not set.