Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/custom/nested-svg-load-events.svg

+<svg id="outersvg" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="reportLoadEvent(this);runTest();">
+  <text y="150" x="20" id="console" onload="reportLoadEvent(this)"/>
+  <script>
+    <![CDATA[
+      if (window.testRunner)
+        testRunner.dumpAsText();
+
+      var results = new Array();
+
+      function reportLoadEvent(el) {
+        results.push(el.id);
+      }
+      
+      function runTest()
+      {
+        var test = document.getElementById("console");
+        if ( results.length != 3 || 
+                         results[0] != "image" ||
+             results[1] != "innersvg" || 
+             results[2] != "outersvg") {
+          test.textContent = "Failed - " + results.join(", ");
+        } else {
+          test.textContent = "Passed";
+        }
+      }
+    ]]>
+  </script>
+  <svg id="innersvg" onload="reportLoadEvent(this)">
+    <image id="image" onload="reportLoadEvent(this)" width="100" height="100" xlink:href="resources/green-checker.png" />
+    <text y="130" x="20">This tests load event dispatching on nested svg elements.</text>
+  </svg>
+</svg>