Unregisters a constructor.

    Args:
      name: A name of the constructor to unregister.

    Raises:
      ValueError: If constructor with specified name has never been registered.

def Unregister(self, name):
    """Unregisters a constructor.

    Args:
      name: A name of the constructor to unregister.

    Raises:
      ValueError: If constructor with specified name has never been registered.
    """
    precondition.AssertType(name, Text)

    try:
      del self._constructors[name]
    except KeyError:
      raise ValueError("Constructor with name '%s' is not registered" % name)