clean up TreeWalker.pop() (#2195)

Remove superfluous parameter.

lib/transform.js

@@ -70,7 +70,7 @@ TreeTransformer.prototype = new TreeWalker;
                     if (y !== undefined) x = y;
                 }
             }
-            tw.pop(this);
+            tw.pop();
             return x;
         });
     };