Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/dynamic-updates/script-tests/SVGUseElement-svgdom-href2-prop.js

 // [Name] SVGUseElement-svgdom-href2-prop.js
 // [Expected rendering result] A use element first with an external then with an internal referenced document - and a series of PASS messages
 
 description("Tests dynamic updates of the 'href' property of the SVGUseElement object")
 createSVGTestCase();
 
 var defsElement = createSVGElement("defs");
 rootSVGElement.appendChild(defsElement);
 
 var useElement = createSVGElement("use");
 useElement.setAttribute("x", "10");
 useElement.setAttribute("y", "10");
-useElement.setAttribute("externalResourcesRequired", "true");
 useElement.setAttribute("onload", "externalLoadDone()");
 
 var rectElement = createSVGElement("rect");
 rectElement.setAttribute("id", "MyRect");
 rectElement.setAttribute("x", "0");
 rectElement.setAttribute("y", "0");
 rectElement.setAttribute("width", "64");
 rectElement.setAttribute("height", "64");
 rectElement.setAttribute("fill", "green");
 
 defsElement.appendChild(rectElement);
 
 rootSVGElement.setAttribute("height", "200");
 rootSVGElement.appendChild(useElement);
 
 function repaintTest() {
     // Start loading external resource, wait for it, then switch back to internal.
     useElement.setAttributeNS(xlinkNS, "xlink:href", "../custom/resources/rgb.svg#R");
 }
 
 function externalLoadDone() {
     useElement.href.baseVal = "#MyRect";
     shouldBeEqualToString("useElement.href.baseVal", "#MyRect");
 
     completeTest();
 }
 
 var successfullyParsed = true;