Allow use of Moodle built-in PHPMailer via phpmailer.dir config option.
Removed ununsed overwrite option.  Zip file can only be created, not overwritten.
Removed our local copy of PHPMailer, since we can now configure the cron.php script to use the Moodle built-in version, or any other standalone version.  See the phpmailer.dir config option.