Remove useless TODO in Field.set_client

Parent is managed in changed call.

src/model.js

@@ -1198,7 +1198,6 @@
                 JSON.stringify(this.get(record))) {
                 record._changed[this.name] = true;
                 this.changed(record).done(function() {
-                    // TODO parent
                     record.validate(null, true).then(function() {
                         record.group.changed().done(function() {
                             var root_group = record.group.root_group();