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: Ic69ecdf3e58aa954bb348838fbbd64c39974a3c8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>