Remove custom FormData.append() bindings code.

Source/core/html/FormData.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 15 matching lines @@
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     WillBeGarbageCollected,
     Constructor([Default=Undefined] optional HTMLFormElement form),
     ImplementedAs=DOMFormData
 ] interface FormData {
-    // void append(DOMString name, DOMString value);
-    // void append(DOMString name, Blob value, optional DOMString filename);
-    [Custom] void append([Default=Undefined] optional DOMString name,
-                         [Default=Undefined] optional DOMString value,
-                         [Default=Undefined] optional DOMString filename);
+    void append(DOMString name, Blob value, optional DOMString filename);
+    void append(DOMString name, DOMString value);
 };