Remove childEvent override

Using qobject_cast on the child in a ChildAdded handler doesn't work, as
the event is sent (not posted) by the QObject constructor (so the vtable
is always QObject's).

As we add all children to the chart in the componentComplete handler we
can just remove this effectively dead code.

Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV707
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV708
Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: I1789f146e2a7e00ba0115161d32ed123bc4afcdb
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>