Refactor lastEventIsSpoiler, remove lastEvent() call that isn't needed

This is only called after we already get an event with lastEvent() so
doing it again is useless. Instead, we should refactor it to behave like
similar functions (e.g. isEventHighlighted.)

src/libneochat/models/roomlistmodel.cpp

@@ -248,7 +248,7 @@ QVariant RoomListModel::data(const QModelIndex &index, int role) const
     }
     if (role == SubtitleTextRole) {
         const auto lastEvent = room->lastEvent(m_hiddenFilter);
-        if (lastEvent == nullptr || room->lastEventIsSpoiler()) {
+        if (lastEvent == nullptr || room->isEventSpoiler(lastEvent)) {
             return QString();
         }
         return EventHandler::subtitleText(room, lastEvent);