Revert "window: Reinstate logic for should_use_csd()"

This reverts commit f043fd532d.

The whole point of having can_use_csd() return TRUE when GTK_CSD is
unset is to use it inside enable_csd().

https://bugzilla.gnome.org/show_bug.cgi?id=751140

gtk/gtkwindow.c

@@ -4079,7 +4079,8 @@ gtk_window_can_use_csd (GtkWindow *window)
 
   csd_env = g_getenv ("GTK_CSD");
 
-  return (g_strcmp0 (csd_env, "1") == 0);
+  /* If GTK_CSD is unset we default to CSD support */
+  return csd_env == NULL || (strcmp (csd_env, "1") == 0);
 }
 
 static void