我想摆脱运行迁移时收到的以下消息：
The following content types are stale and need to be deleted:
appname | modelname
Any objects related to these content types by a foreign key will also
be deleted. Are you sure you want to delete these content types?
在迁移中对删除或不删除的决定进行编码似乎更干净。
在迁移过程中检测和删除过时的ContentType的最干净方法是什么？如果自动创建 ContentType，为什么不自动完成？