xwayland: drop wlr_xwayland_surface.events.set_pid

The PID of an X11 window cannot change.

This is a remnant from the days when we queried the PID with a
window property, instead of using XRes.

include/wlr/xwayland/xwayland.h

@@ -156,7 +156,6 @@ struct wlr_xwayland_surface {
 		struct wl_signal set_class;
 		struct wl_signal set_role;
 		struct wl_signal set_parent;
-		struct wl_signal set_pid;
 		struct wl_signal set_startup_id;
 		struct wl_signal set_window_type;
 		struct wl_signal set_hints;

xwayland/xwm.c

@@ -177,7 +177,6 @@ static struct wlr_xwayland_surface *xwayland_surface_create(
 	wl_signal_init(&surface->events.set_role);
 	wl_signal_init(&surface->events.set_title);
 	wl_signal_init(&surface->events.set_parent);
-	wl_signal_init(&surface->events.set_pid);
 	wl_signal_init(&surface->events.set_startup_id);
 	wl_signal_init(&surface->events.set_window_type);
 	wl_signal_init(&surface->events.set_hints);
@@ -627,7 +626,6 @@ static void read_surface_client_id(struct wlr_xwm *xwm,
 		return;
 	}
 	xsurface->pid = *pid;
-	wl_signal_emit_mutable(&xsurface->events.set_pid, NULL);
 	free(reply);
 }