I was fiddling trying to get uMatrix working and accidentally posted what should have been a comment to another answer, but was actually posted as an answer. It was noticed by a reviewer and deleted as a "low quality" answer (which it certainly was). Now I want to permanently delete it, but can't figure out how to do that.
It is already permanently deleted. You can just still see it because you are the post's owner, but other users cannot see it. "Deleted" content on these sites isn't really deleted, it is simply not shown to users unless they have at least 10,000 reputation or, as in this case, if they are the post's author.
So don't worry about it, it is deleted, it isn't bothering anyone and there's nothing wrong with leaving it.