gdkwindow: Deprecate gdk_window_flush

The function does nothing.

gtk/gtkmain.c

@@ -1663,13 +1663,6 @@ gtk_main_do_event (GdkEvent *event)
         }
       else
         {
-          /* The app may paint with a previously allocated cairo_t,
-           * which will draw directly to the window. We can't catch cairo
-           * draw operations to automatically flush the window, thus we
-           * need to explicitly flush any outstanding moves or double
-           * buffering
-           */
-          gdk_window_flush (event->any.window);
           gtk_widget_send_expose (event_widget, event);
         }
       break;