Quick fix for not working history in demobrowser

Current implementation of history manager seems to
be not straight forward. To display history we use QList to keep
history items plus model adapter in form of HistoryModel
(QAbstractTableModel) plus HistoryFilterModel to avoid
duplication, plus HistoryTreeModel to make a tree view with
split for dates, plus TreeProxyModel on top to enable
sorting. This approach unfortunately falls apart when items
should be deleted from the model by treeView. This ends
badly with corrupted cached values.

This fix removes history items using history manager.
It also abandons HistoryTreeModel since frequent sourceReset calls make
it unusable. If split for dates it desired the better approach
would be to implement history tree model already at HistoryManager
level to avoid unmaintainable code.

Task-number: QTBUG-49913
Task-number: QTBUG-50255
Change-Id: Ic9cebb52b623bd453119e11b5e907eaa90609e34
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>