mscsplit: directly access gsm_network backpointer from gsm_subscriber_connection

The previous commit added a network backpointer to gsm_subscriber_connection.
Use it wherever it makes sense, to skip the step through the bts structure.

In some places, remove local variables that become unused.

Change-Id: I34537025986713291e14c8212a81539b497befd4
diff --git a/openbsc/src/libmsc/transaction.c b/openbsc/src/libmsc/transaction.c
index a750362..dba4bed 100644
--- a/openbsc/src/libmsc/transaction.c
+++ b/openbsc/src/libmsc/transaction.c
@@ -37,7 +37,7 @@
 				   uint8_t proto, uint8_t trans_id)
 {
 	struct gsm_trans *trans;
-	struct gsm_network *net = conn->bts->network;
+	struct gsm_network *net = conn->network;
 	struct gsm_subscriber *subscr = conn->subscr;
 
 	llist_for_each_entry(trans, &net->trans_list, entry) {
@@ -155,7 +155,7 @@
 {
 	struct gsm_trans *trans;
 
-	llist_for_each_entry(trans, &conn->bts->network->trans_list, entry)
+	llist_for_each_entry(trans, &conn->network->trans_list, entry)
 		if (trans->conn == conn)
 			return 1;