cosmetic: remove unused scall_signal_data.subscr

Doesn't make sense to switch this to struct vlr_subscr when it isn't used at
all. So let's remove it.

Change-Id: Ifa5901f8bf1aed3981841d24d4ec8d659f3de7a9
diff --git a/openbsc/include/openbsc/signal.h b/openbsc/include/openbsc/signal.h
index 8f27b38..c862b21 100644
--- a/openbsc/include/openbsc/signal.h
+++ b/openbsc/include/openbsc/signal.h
@@ -151,7 +151,6 @@
 };
 
 struct scall_signal_data {
-	struct gsm_subscriber *subscr;
 	struct gsm_subscriber_connection *conn;
 	void *data;
 };