input: Remove wl_seat destroy signal

We already have one on weston_seat.
diff --git a/src/clipboard.c b/src/clipboard.c
index 7db5d04..7f8d93b 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -270,7 +270,7 @@
 
 	wl_signal_add(&seat->seat.selection_signal,
 		      &clipboard->selection_listener);
-	wl_signal_add(&seat->seat.destroy_signal,
+	wl_signal_add(&seat->destroy_signal,
 		      &clipboard->destroy_listener);
 
 	return clipboard;