remove _parent from nodes in traverse.removeProperties

lib/6to5/traverse/index.js

@@ -87,6 +87,7 @@ traverse.removeProperties = function (tree) {
   var clear = function (node) {
     delete node.extendedRange;
     delete node._scopeIds;
+    delete node._parent;
     delete node.tokens;
     delete node.range;
     delete node.start;