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/rrlp.c b/openbsc/src/libmsc/rrlp.c
index 161456a..e695daa 100644
--- a/openbsc/src/libmsc/rrlp.c
+++ b/openbsc/src/libmsc/rrlp.c
@@ -40,7 +40,7 @@
 
 static int send_rrlp_req(struct gsm_subscriber_connection *conn)
 {
-	struct gsm_network *net = conn->bts->network;
+	struct gsm_network *net = conn->network;
 	const uint8_t *req;
 
 	switch (net->rrlp.mode) {