Normally you can delete them safely, as these are content types where no model exists anymore.
The only thing you should be aware of is, that if you had other models pointing to the
ContentType model via a
ForeignKey these objects will be deleted as well. If you didn’t set any foreign keys to
ContentType at all nothing bad can happen, if you did evaluate if you would still need the objects that have a foreign key to the content type you are about to delete (if there are any).