use deleted flag for proper array item exclude in getContent values

getContent get information about arrays from dom, so parent get notification
before element deleted. use deleted flag solve this issue.

jsonform/gadget_json_generated_form_child.js

@@ -1833,6 +1833,7 @@
         );
       }
       tasks.push(g.notifyInvalid([], g.element.getAttribute("data-gadget-scope")));
+      g.props.deleted = true;
       return new RSVP.Queue()
         .push(function () {
           return RSVP.all(tasks);
@@ -2315,6 +2316,9 @@
 
     .declareMethod('getContent', function () {
       var g = this;
+      if (g.props.deleted) {
+        return;
+      }
       return getFormValuesAsJSONDict(g)
         .push(function (data) {
           if (g.props.updatePropertySelectors) {