Remove hack to prevent image resize when viewer is fullscreen.

When the ViewerWidget was set to show in fullscreen mode, the old
behaviour is to resize to the screen size preemptively to avoid one
scale operation.
The known dowsides are:

• Going to fullscreen and back again moves the viewer window
• Presumably, this is not ideal for wayland interoperability (Though that's mostly heresay on my part)

Since I can't reproduce any difference in behaviour on a current system
with this page disabled or enabled, I've come to the conclusion that
the reasoning for adding it no longer holds...