This is a bug.
The program shouldn't be trying to unlink if you are manually entering the filename... The FTP owner is not the same as the web/PHP owner, so PHP cannot delete those files.
If you're up to it, to disable the error message/warning you can modify the edituser.php file to say @unlink($filename) instead of unlink($filename) on that line (863). This issue has been fixed for future updates.
Regards