Error Message for Group Double Opt-in give wrong instructions

Steps to reproduce:

1. Create a Profile.
2. Create a Contact:Group(s) field only. Click Save. The system will respond:
"Your site is currently configured to require double-opt in when users join (subscribe) to Group(s) via a Profile form. In this mode, you need to include an Email field in a Profile BEFORE you can add the Group(s) field. This ensures that an opt-in confirmation email can be sent. Your site administrator can disable double opt-in by adding this line to the CiviCRM settings file: define( 'CIVICRM_PROFILE_DOUBLE_OPTIN' , 0 );"
3. Add the suggested line to civicrm.settings.php
4. Try to save the Gorup field again.

Expected results:
User should now be permitted to save the field.

Actual results:
No change. Same warning message. CiviCRM does not appear to use this constant at all.

Notes:

The error message is set in /CRM/UF/Form/Field.php:890

The actual solution to the issue is to modify the setting at the URL 'civicrm/admin/setting/preferences/mailing'