Remove support for the externalResourcesRequired attribute.

LayoutTests/http/tests/svg/dynamic-image-root.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+  <script>
+          if (window.testRunner) {
+                testRunner.waitUntilDone();
+                testRunner.dumpAsText();
+          }
+
+          window.onload = function() {
+        if (window.testRunner)
+          testRunner.notifyDone();
+          };
+  </script>
+  <text y="170" x="20" id="console">Result: </text>
+  <script>
+      var img = document.createElementNS("http://www.w3.org/2000/svg", "image");
+      img.x.baseVal.value = 20;
+      img.y.baseVal.value = 20;
+      img.width.baseVal.value = 100;
+      img.height.baseVal.value = 100;
+      img.addEventListener("load", loaded, false);
+      img.addEventListener("SVGLoad", loaded, false);
+      img.addEventListener("error", loaded, false);
+      img.addEventListener("SVGError", loaded, false);
+      img.href.baseVal = "http://127.0.0.1:8000/svg/resources/delayCachedLoad.php";
+      document.documentElement.appendChild(img);
+
+      function loaded(evt) {
+                document.getElementById("console").textContent += evt.type + "(" + evt.target + ") ";
+      }
+  </script>
+</svg>