Merge label filter into TransferListSortModel

This also fixes a bug that when label filter contains special symbols
from regex, the label filter may match torrents with multiple different
labels.

src/transferlistwidget.h

@@ -113,7 +113,6 @@ private:
   TransferListDelegate *listDelegate;
   TorrentModel *listModel;
   TransferListSortModel *nameFilterModel;
-  QSortFilterProxyModel *labelFilterModel;
   QBtSession* BTSession;
   MainWindow *main_window;
   QShortcut *editHotkey;