window: remove the small screen code

Because the old layout was never meant to scale well on low
resolution displays, we had to introduce code that adapts the
window to be usable on low res screens.

The problem with this code is that it still doesn't scale down
very well for really low resolution screens. Partially because
of the layout itself (which, again, was never meant to), partially
because the panels request a size bigger than e.g. 720x480. Now,
this is an important feature and we need to support that low
resolution by default.

To push this constraint forward, remove the code that managed a
custom mode for low resolution screens. From now on, the Control
Center shall be adapted to scale well on any screen sizes.

https://bugzilla.gnome.org/show_bug.cgi?id=766922