I'd like to get rid of the text that precedes the message which says "Message from sender". I see in print_mail.inc, the message gets rendered as:

$params['message'] = t('Message from sender') . ':<br /><br /><em>' . nl2br(check_plain($form_state['values']['txt_message'])) . '</em>';

I tried a form alter in my theme's template.php but it had no effect and I simply got an error.

function MYTHEME_form_alter(&$form, &$form_state, $form_id) {
    if ($form_id == 'print_mail_form') {
      $params = array();
      $params['message'] = nl2br(check_plain($form_state['values']['txt_message']));
    }
}

I'm pretty sure this is not the right way to do it but I didn't think there was a UI setting for that either or if this is even possible.