The use of the startup ID is now twofold, we reply back with it to end any
corresponding startup notification, but we also use it on
gtk_surface1.request_focus to acknowledge that the activation might raise
the corresponding window.
We should preserve the startup ID for the second to work properly, so avoid
clearing it here. It is inconsequential if the underlying
gtk_shell1.set_startup_id request happens multiple times on no longer existing
startup IDs, so don't bother preventing that from happening.
Closes: https://gitlab.gnome.org/GNOME/gtk/issues/1754
ed9a6f755d