The issue in QtQuick3D blocking effect previews (QTBUG-86616) is not
going to be fixed in Qt 5 anymore, and Effect was moved to QtQuick3D
from QtQuick3D.Effects in Qt 6, so this can't be written in Qt 5 and
Qt 6 compatible manner anyway. Removed the support for now as it needs
to be implemented differently if we want to enable effect previews in
the future.
Change-Id: Ic786608572c3350e22dab75366cb93586c29459a
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>