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" />
+    <image 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>