Remove support for the externalResourcesRequired attribute.

LayoutTests/svg/repaint/image-with-clip-path.svg

 <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="startTest()" width="1000" height="600">
     <script xlink:href="../../fast/repaint/resources/repaint.js" type="text/javascript"></script>
     <text y="-10">Bug 76559: All red should disappear</text>
     <g id="g" transform="translate(0, 0)">
         <defs>
             <clipPath id="p">
                 <rect id="r" x="-10" y="-10" width="20" height="20" />
             </clipPath>
         </defs>
         <image x="-30" y="-30" width="60" height="60" clip-path="url(#p)" xlink:href=""/>
@@ skipping 11 matching lines @@
 
     var r = document.querySelector('#r');
     var g = document.querySelector('#g');
     var i = document.querySelector('image');
     var update = function(offset, size, img) {
         r.setAttribute('x', -size / 2);
         r.setAttribute('y', -size / 2);
         r.setAttribute('width', size);
         r.setAttribute('height', size);
         g.setAttribute("transform","translate(" + offset + ",50)");
-        i.setAttribute("externalResourcesRequired", "true");
         i.setAttributeNS('http://www.w3.org/1999/xlink', 'href', img);
     };
 
     if (window.testRunner) {
         testRunner.dumpAsTextWithPixelResults();
         testRunner.waitUntilDone();
     }
 
     function finishTest() {
         if (window.testRunner)
             testRunner.notifyDone();
     }
 
     function nextStep() {
         i.onload = function() { setTimeout(finishTest, 0); };
         update(200, 50, green);
     }
 
     function repaintTest() {
         i.onload = function() { setTimeout(nextStep, 0); };
         update(50, 100, red);
     }
 </script>
 </svg>