Delete admin_created_user_subject() rather than deprecate

As it was never used as anything more than a callback to a filter before the MU merge, and is only available in user-new.php in multisite, it is safe to remove this function entirely.

Fixes #29915


git-svn-id: https://develop.svn.wordpress.org/trunk@30176 602fd350-edb4-49c9-b593-d223f7449a82