Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/custom/loadevents-capturing.svg

-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
-  <script>
-    <![CDATA[
-      if (window.testRunner)
-        testRunner.dumpAsText();
-
-      var results = [ ];
-      function handler(event)
-      {
-        results.push(event.target.localName);
-      }
-      document.documentElement.addEventListener('load', handler, true);
-
-      function reportResults()
-      {
-        var console = document.getElementById("console");
-        var expected = [ "image", "text", "text", "text", "g", "svg" ];
-        for (var i = 0; i < expected.length; ++i) {
-            if (results[i] != expected[i]) {
-              console.appendChild(document.createTextNode("FAIL: " + i + " should be " + expected[i] + " but instead is " + results[i] + ".\n"));
-              return;
-            }
-        }
-        console.appendChild(document.createTextNode("Passed"));
-      }
-      document.documentElement.addEventListener('load', reportResults);
-    ]]>
-  </script>
-  <g>
-    <image externalResourcesRequired="false" id="image" width="100" height="100" xlink:href="resources/green-checker.png" />
-    <text y="130" x="20">This tests that load dispatching works when there are no</text>
-    <text y="150" x="20">direct listeners, but there are capturing event listeners on an ancestor. Bug 16447</text>
-    <text y="170" x="20" id="console" />
-  </g>
-</svg>