Remove compatibility for GTK+-2.0

While the Meego developers agreed to switching mutter to GTK+-3.0
unconditionally a while ago, Canonical used a GTK+-2.0 build for their
Unity project. As Canonical now announced a switch to compiz as their
window manager, there is no longer a reason to maintain GTK+-2.0
compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=633133

src/tools/mutter-window-demo.c

@@ -24,8 +24,6 @@
 #include <X11/Xatom.h>
 #include <unistd.h>
 
-#include "gtk-compat.h"
-
 static GtkWidget* do_appwindow (void);
 
 static gboolean aspect_on;