Unset a value in the configuration.

def unset_value(self, key):
        # type: (str) -> None
        """Unset a value in the configuration.
        """
        self._ensure_have_load_only()

        if key not in self._config[self.load_only]:
            raise ConfigurationError("No such key - {}".format(key))

        fname, parser = self._get_parser_to_modify()

        if parser is not None:
            section, name = _disassemble_key(key)

            # Remove the key in the parser
            modified_something = False
            if parser.has_section(section):
                # Returns whether the option was removed or not
                modified_something = parser.remove_option(section, name)

            if modified_something:
                # name removed from parser, section may now be empty
                section_iter = iter(parser.items(section))
                try:
                    val = six.next(section_iter)
                except StopIteration:
                    val = None

                if val is None:
                    parser.remove_section(section)

                self._mark_as_modified(fname, parser)
            else:
                raise ConfigurationError(
                    "Fatal Internal error [id=1]. Please report as a bug."
                )

        del self._config[self.load_only][key]