Use built-in sorting so that dynamic updates work

src/ReadReceiptsModel.cpp

@@ -125,11 +125,6 @@ ReadReceiptsProxy::ReadReceiptsProxy(QString event_id, QString room_id, QObject
 {
         setSourceModel(&model_);
         setSortRole(ReadReceiptsModel::RawTimestamp);
-}
-
-bool
-ReadReceiptsProxy::lessThan(const QModelIndex &source_left, const QModelIndex &source_right) const
-{
-        // since we are sorting from greatest to least timestamp, return something that looks totally backwards!
-        return source_left.data().toULongLong() > source_right.data().toULongLong();
+        sort(0, Qt::DescendingOrder);
+        setDynamicSortFilter(true);
 }

src/ReadReceiptsModel.h

@@ -63,8 +63,6 @@ public:
         QString eventId() const { return event_id_; }
         QString roomId() const { return room_id_; }
 
-        bool lessThan(const QModelIndex &source_left, const QModelIndex &source_right) const;
-
 private:
         QString event_id_;
         QString room_id_;