[Staleness] Fix removed object error and clean up (#6241)

* fixing error from plots when removing swg and making methods and props private for swg staleness provider

* removing unsubscribes from destroy hooks if the item has been removed already and reverting an unneccesary change

* checking for undefined staleness response

* removed un-neccesary code

src/plugins/LADTable/components/LadTableSet.vue

@@ -218,6 +218,7 @@ export default {
                 this.stalenessSubscription[keystring].unsubscribe();
                 this.stalenessSubscription[keystring].stalenessUtils.destroy();
                 this.handleStaleness(keystring, { isStale: false }, SKIP_CHECK);
+                delete this.stalenessSubscription[keystring];
             };
         },
         handleStaleness(id, stalenessResponse, skipCheck = false) {