So far when the PlasmaWindowManagement went down the PlasmaWindowModel
deleted all windows. This is not needed as the PlasmaWindow has the
PlasmaWindowManagement as parent, so it will be destroyed anyway.
The delete in PlasmaWindowModel risks that other signals connecting
to the same signal will operate on deleted data, which they did not
expect. So it's better to not delete here and let the later auto
delete handle it.