gdk/monitor: Remove gdk_monitor_work_area and GdkMonitor::work-area
It's not a portable API, so remove it. The corresponding backend specific functions are still available, if they were implemented, e.g. gdk_macos_monitor_get_workarea() and gdk_x11_monitor_get_workarea().
This makes things behave slightly worse on e.g. X11 or Windows, as the work area isn't used anymore, meaning windows might end up too big. The intention is to fix this differently, but take first step and remove the work area from the common API.