Designer: remove dead code in QStackedWidgetPreviewEventFilter::updateButtonToolTip()

The local variable 'className' is no longer used, since at some point
in time, it was replaced with the query stackedClassName(), a file-
static function.

So remove the code that sets it.

Change-Id: I2ee7dedc66acfa10949b628499c52f87dab02e7a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
(cherry picked from qttools/c98012e7a6b6287ec13b8ca4a4d08ed584f7ee8a)