Bug 816380 followup. Remove unnecessary code.

dom/bindings/Bindings.conf

@@ -810,7 +810,6 @@ addExternalIface('HitRegionOptions', nativeType='nsISupports')
 addExternalIface('LockedFile')
 addExternalIface('MediaStream')
 addExternalIface('NamedNodeMap')
-addExternalIface('nsISupports', nativeType='nsISupports')
 addExternalIface('OutputStream', nativeType='nsIOutputStream',
                  notflattened=True)
 addExternalIface('PaintRequest')

dom/webidl/XPathEvaluator.webidl

@@ -4,7 +4,6 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-interface nsISupports;
 interface XPathExpression;
 interface XPathNSResolver;
 interface XPathResult;