gtk: drop gtk_widget_set_double_buffered call

Dunno why it is here.  Removing it seems to have no ill side effects.
It is depricated in 3.14+.  In some cases it has no effect since 3.10
according to the docs:

https://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-set-double-bufferedSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/gtk.c

@@ -1666,7 +1666,6 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
                           GDK_LEAVE_NOTIFY_MASK |
                           GDK_SCROLL_MASK |
                           GDK_KEY_PRESS_MASK);
-    gtk_widget_set_double_buffered(vc->gfx.drawing_area, FALSE);
     gtk_widget_set_can_focus(vc->gfx.drawing_area, TRUE);
 
     vc->type = GD_VC_GFX;