gdk/wayland: Do not use xdg_wm_base.ping serials as "user input" serials

Ping/pong serials are not meant to be interpreted as user input serials
(e.g. those given back later to the compositor on grabs). As a matter
of fact, Mutter uses a different count (i.e. timestamps) in these, so
using these serials may confuse the compositor into denying certain
operations like DnD.

gdk/wayland/gdkdisplay-wayland.c

@@ -130,10 +130,6 @@ xdg_wm_base_ping (void               *data,
                   struct xdg_wm_base *xdg_wm_base,
                   uint32_t            serial)
 {
-  GdkWaylandDisplay *display_wayland = data;
-
-  _gdk_wayland_display_update_serial (display_wayland, serial);
-
   GDK_NOTE (EVENTS,
             g_message ("ping, shell %p, serial %u\n", xdg_wm_base, serial));