fix(query-subs): Fix onLastCallbackRemoved overriding with correct class syntax

- Using arrow function syntax for a class method that is meant to be overriden
will prevent it from being properly overriden
- This caused onLastCallbackRemoved not to be called

src/flux/models/query-subscription.es6

@@ -67,7 +67,7 @@ export default class QuerySubscription {
     }
   }
 
-  onLastCallbackRemoved = () => {
+  onLastCallbackRemoved() {
 
   }
 

src/flux/models/unread-query-subscription.es6

@@ -42,7 +42,7 @@ export default class UnreadQuerySubscription extends MutableQuerySubscription {
     this._query = buildQuery(this._categoryIds).limit(limit).offset(offset);
   }
 
-  onLastCallbackRemoved = () => {
+  onLastCallbackRemoved() {
     this._unlisten();
   }
 }