Use jQuery and you can do it with one line of code:
$('#sort_table tbody tr:not([class!=""]):first').remove ();
See "Select elements without any class".
The complete Greasemonkey script would be like this:
// ==UserScript==
// @name _Remove the first body row that ain't got no class
// @include http://YOUR_SERVER.COM/YOUR_PATH/*
// @require http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js
// @grant GM_addStyle
// ==/UserScript==
/*- The @grant directive is needed to work around a design change
introduced in GM 1.0. It restores the sandbox.
*/
$('#sort_table tbody tr:not([class!=""]):first').remove ();