In sendmessage.inc.php $RawBody is initialized on line 144:

$RawBody = $Body;

The PMN_CONTENT ones are adding different things like who it's from, the url to the post, etc. The call to add_post is where it's actually added to the email on line 308 in that script and you could just comment that out:

Code
            $mailer->add_post($fromname,$Subject,array(),$Message,$RawBody);