Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/custom/svg2-loadevents-normal-displaynone.svg

 <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="reportLoadEvent(this);runTest();">
   <script>
     <![CDATA[
       if (window.testRunner)
         testRunner.dumpAsText();
 
       var results = new Array();
 
       function reportLoadEvent(el) {
         results.push(el.localName);
       }
       function runTest()
       {
         var test = document.getElementById("console");
-        if ( results.length != 4 || results[0] != "image" ||
-             results[1] != "text" || results[2] != "g" ||
-             results[3] != "svg") {
+        if ( results.length != 2 || results[0] != "image" ||
+             results[1] != "svg") {
           test.appendChild(document.createTextNode("Failed"));
         } else {
           test.appendChild(document.createTextNode("Passed"));
         }
       }
     ]]>
   </script>
   <g onload="reportLoadEvent(this)">
-    <image externalResourcesRequired="false" id="image" onload="reportLoadEvent(this)" width="100" height="100" xlink:href="resources/green-checker.png" />
-    <text y="130" x="20">This tests normal load dispatching order. Bug 16447</text>
+    <image display="none" id="image" onload="reportLoadEvent(this)" width="100" height="100" xlink:href="resources/green-checker.png" />
+    <text y="130" x="20">This tests normal load dispatching order and an image that does not render(display=none).</text>
     <text y="150" x="20" id="console" onload="reportLoadEvent(this)"/>
   </g>
 </svg>