reftests: Remove window-show-contents-on-map.ui test

reftests: Remove window-show-contents-on-map.ui test

It tested that a widget shown after the window was mapped deals with
window resizing appropriately when doing the follow-up allocation. Now,
doing this only allocates both at the same time, as allocation happens
during the frame dispatch.

To do the equivalent now, one would have to write code that shows the
label after the first frame was drawn, and that's not possible via a
reftest, so lets remove it.