gdk-win32: Clean Up A Bit

gdk-win32: Clean Up A Bit

As GLib dropped Windows 2000 support some time ago [1][2], and the current
git master already depends on a GLib release that is well after it dropped
Windows 2000 support, clean up the code a bit as we are assured that the
code will run on XP and later, plus, we have dropped XP support during
this cycle with commit b85f0cc.

https://bugzilla.gnome.org/show_bug.cgi?id=741849

[1]: https://git.gnome.org/browse/glib/commit/?id=80c24d36f2525d83e458ebbdf62fdbd085945a02
[2]: https://git.gnome.org/browse/glib/commit/?id=731b46990896665a8107535080bb075a6e18b6f7