input: Remove wl_seat destroy signal

We already have one on weston_seat.
diff --git a/src/shell.c b/src/shell.c
index de7e9ba..b7ee576 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -471,7 +471,7 @@
 
 	state->seat_destroy_listener.notify = focus_state_seat_destroy;
 	state->surface_destroy_listener.notify = focus_state_surface_destroy;
-	wl_signal_add(&seat->seat.destroy_signal,
+	wl_signal_add(&seat->destroy_signal,
 		      &state->seat_destroy_listener);
 	wl_list_init(&state->surface_destroy_listener.link);