When you are using attachURL - iframe element created automatically, which produce scrollbars automatically if content inside area is too big.
There is no any way to disable such scrolls through API.
Can be done by changes in code
dhtmlxwindow.js
if (type == “url”) {
var fr = document.createElement(“IFRAME”);
fr.scrolling=“no”;//this line can be added
fr.frameBorder = 0;