Removes an option from the manager.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define
                any option.

def unregister_option(self, key, subkey):
        """Removes an option from the manager.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define
                any option.
        """
        if not self.open:
            return

        key, subkey = _lower_keys(key, subkey)
        _entry_must_exist(self.gc, key, subkey)

        self.gc = self.gc[~((self.gc['k1'] == key) &
                            (self.gc['k2'] == subkey))]