Clean extra files/directories specified by get_extra_paths()

def clean_extra(self):
        """Clean extra files/directories specified by get_extra_paths()"""
        extra_paths = self.get_extra_paths()
        for path in extra_paths:
            if not os.path.exists(path):
                continue
            if os.path.isdir(path):
                self._clean_directory(path)
            else:
                self._clean_file(path)