Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/dom/SVGScriptElement/script-reexecution.svg

 <?xml version="1.0"?>
 <svg width="100%" height="100%" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xhtml="http://www.w3.org/1999/xhtml" onload="startTest()">
 <defs>
 <script type="text/javascript">
 function pass() {
     var contentElement = document.getElementById(test);
 
     if (contentElement.textContent == 'PASS') {
         contentElement.textContent = 'FAIL';
     } else if (contentElement.textContent == 'FAIL') {
@@ skipping 14 matching lines @@
     document.getElementsByTagName("defs")[0].removeChild(e1);
 
     test = "p2";
     document.getElementsByTagName("defs")[0].appendChild(e1);
     document.getElementsByTagName("defs")[0].removeChild(e1);
 
     test = "p3";
     message = "PASS";
 
     e3 = document.createElementNS("http://www.w3.org/2000/svg", "svg:script");
-    e3.externalResourcesRequired.baseVal = true; // Otherwhise SVGLoad event will be fired immediately.
     e3.href.baseVal = "resources/script-reexecution.js";
-    e3.onload = function() { finish(); }
     document.getElementsByTagName("defs")[0].appendChild(e3);
 }
 
 function finish() {
     test = "p4";
     message = "FAIL";
 
     e3.onload = "";
     document.getElementsByTagName("defs")[0].removeChild(e3);
     document.getElementsByTagName("defs")[0].appendChild(e3);
 
     if (window.testRunner)
         testRunner.notifyDone();
 }
 </script>
 </defs>
 <foreignObject>
 <xhtml:p>Created script element, script data passed as text content, appended: <xhtml:span id="p1">FAIL</xhtml:span></xhtml:p>
 <xhtml:p>Removed element, readd element, remove again, script shouldn't have executed:<xhtml:span id="p2">PASS</xhtml:span></xhtml:p>
 <xhtml:p>Created script element, loading external script content, appended: <xhtml:span id="p3">FAIL</xhtml:span></xhtml:p>
 <xhtml:p>Removed element, readd element, script shouldn't have executed: <xhtml:span id="p4">PASS</xhtml:span></xhtml:p>
 </foreignObject>
 </svg>