window: remove the fixed width request code

Having a fixed width is bad for various reasons. First, it's
harder to deal with low resolution screens, which was fixed
by adding an entire new mode just to deal with it. Second,
because it may not scale well for big resolutions.

Fix that by removing the code that handles the fixed width.
The next commit will completely remove the code that manages
small screens, in the hope that the new layout will handle
both cases well by design.

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