Remove support for the externalResourcesRequired attribute.

LayoutTests/http/tests/svg/dynamic-image-after-docload.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="loaded(evt); testDone();">
+  <text y="170" x="20" id="console">Test didn't run.</text>
+  <script>
+      if (window.testRunner) {
+        testRunner.waitUntilDone();
+        testRunner.dumpAsText();
+      }
+
+      var results = new Array();
+
+      function loaded(evt) {
+        results.push(evt.type + "(" + evt.target + ")");
+      }
+
+      function testDone() {
+        if ( results.length != 2 ||
+             results[0] != "load([object SVGImageElement])" ||
+             results[1] != "load([object SVGSVGElement])") {
+          document.getElementById("console").textContent = "Failed - got " + results.join(", ");
+        } else {
+          document.getElementById("console").textContent = "Passed";
+        }
+
+        if (window.testRunner)
+          testRunner.notifyDone();
+      }
+
+      var img = document.createElementNS("http://www.w3.org/2000/svg", "image");
+      img.setAttribute("x", 20);
+      img.setAttribute("y", 20);
+      img.setAttribute("width", 100);
+      img.setAttribute("height", 100);
+      img.addEventListener("load", loaded, false);
+      img.addEventListener("SVGLoad", loaded, false);
+      img.addEventListener("error", loaded, false);
+      img.addEventListener("SVGError", loaded, false);
+      img.setAttributeNS("http://www.w3.org/1999/xlink", "xlink:href", "http://127.0.0.1:8000/svg/resources/delayCachedLoad.php");
+      document.documentElement.appendChild(img);
+  </script>
+</svg>