Removes an entry from the list of known classes.

    Args:
        class_name: A string with the class name that is to be removed.
    Raises:
        NonexistentMapping if there is no class with the specified class_name.

def RemoveClass(self, class_name):
    """Removes an entry from the list of known classes.

    Args:
        class_name: A string with the class name that is to be removed.
    Raises:
        NonexistentMapping if there is no class with the specified class_name.
    """
    if class_name not in self._class_mapping:
      raise problems.NonexistentMapping(class_name)
    del self._class_mapping[class_name]