So I went looking in the code, and I see that there's an even easier way to deal with it, from v151 onward:
In your language files (create_account.php), simply change the define for EMAIL_SUBJECT from whatever it currently is (probably "welcome to".STORE_NAME or such), to 'n/a', like this:
Code:
define('EMAIL_SUBJECT', 'n/a');
And for admin-copies of those emails, in email_extras, do similarly:
Code:
define('SEND_EXTRA_CREATE_ACCOUNT_EMAILS_TO_SUBJECT', 'n/a');
This way you don't need to alter the code in the "module" files.