SOLVED: I was still logged in as the administrator I wanted to delete. Once I logged in again as the new admin, I was able to delete the old one.
For reasons that aren’t important here, I needed to make a new admin account for myself and delete the old one. I just did this with no problem on the subdomain. On the domain, however, the “delete” link doesn’t appear on my original admin account. I’ve read that this is because that user is the “owner” of the site, but I can’t find anything anywhere that indicates ownership. The site’s administrative email address (under Settings/General) is the same email address as the new admin account.
Why can’t I delete that user?