There's no way to drop the email column without a lot of work, because it's a unique column used by Flarum and many extensions like social logins.
The best course of action is to have an extension insert dummy email addresses and disable outbound email.
I don't think there's any extension that implements this as a complete product. My JWT cookie login extension contains the logic to generate dummy unique email addresses, but it's part of the global login feature if only usernames are used.
