Remove a section, it cannot be the DEFAULT section.

def remove_section(self, section_name):
        """Remove a section, it cannot be the DEFAULT section.
        """
        if section_name == "DEFAULT":
            raise Exception("'DEFAULT' is reserved section name.")

        if section_name in self._sections:
            del self._sections[section_name]
        else:
            raise Exception("Error! cannot find section '%s'.")