input: Remove wl_seat destroy signal

We already have one on weston_seat.
diff --git a/src/text-backend.c b/src/text-backend.c
index 3037fe8..aa91da7 100644
--- a/src/text-backend.c
+++ b/src/text-backend.c
@@ -874,7 +874,7 @@
 				      input_method, bind_input_method);
 
 	input_method->destroy_listener.notify = input_method_notifier_destroy;
-	wl_signal_add(&seat->seat.destroy_signal, &input_method->destroy_listener);
+	wl_signal_add(&seat->destroy_signal, &input_method->destroy_listener);
 
 	seat->input_method = input_method;