Remove Attr child nodes (making Attr a Node, not a ContainerNode)

LayoutTests/fast/dom/import-attribute-node.html

 <p>Test for <a href="http://bugs.webkit.org/show_bug.cgi?id=15650">bug 15650</a>:
 XML attribute nodes are not importable.</p>
 <script>
 if (window.testRunner)
   testRunner.dumpAsText();
 
 try {
   var xmld = new DOMParser().parseFromString( "<foo test='baz'/>", "text/xml");
   var xmld2 = new DOMParser().parseFromString("<bar/>", "text/xml");
 
   var srcElem = xmld.documentElement;
   var dstElem = xmld2.documentElement;
 
   var importedNode = xmld2.importNode(srcElem.attributes["test"], false);
-  if (importedNode.firstChild.nodeValue != "baz")
-    throw "wrong imported attribute child: '" + importedNode.firstChild.nodeValue + "'";
 
   dstElem.setAttributeNode(importedNode);
   if (dstElem.getAttribute("test") != "baz")
     throw "wrong imported attribute value: '" + dstElem.getAttribute("test") + "'";
 
   document.write("<div>SUCCESS</div>");
 } catch (ex) {
   document.write("<div>" + ex + "</div>");
 }
 </script>