Yes, it's in external.php. Just look into the case where you want to remove it (watch out: there's a difference between RSS and RSS2). If you want to remove it in RSS2 you just pasted the right code snippet. To remove the newpost-parameter just null the first array in the link-tag:
PHP Code:
$xml->add_tag('link', $vbulletin->options['bburl'] . '/' . fetch_seo_url('thread|nosession|js', $thread, array(), array(), false, true);
I did quiet some editing within the external.php. So if you have more questions, just ask.