qt: remove dubious and non functional call

As no videoWidget is ever inserted into the stackWidgetsSizes QMap, a
default QSize is provided with width and height = -1. These uninitialized
values are then propagated and end up as a no op at some point.

Yet, on Wayland, this -1 default values can end up being transmitted to
the Wayland server, that treats them as invalid values (protocol error).
At least the Gnome server(mutter) does.

This fix simply removes the call.