sms_next_rp_msg_ref(): use direct pointer to next_rp_ref counter

libbsc and libmsc will have separate subscriber connection structs. Hence don't
rely on gsm_subscriber_connection, but work on a direct pointer to the counter
for the next RP reference.

The only very thin function in gsm_04_11_helper.c thus becomes obsolete: drop
the entire file.

Change-Id: I2a2e9ba6a981a385d1f8f07acbe03536ffed0072
diff --git a/openbsc/src/libmsc/gsm_04_11.c b/openbsc/src/libmsc/gsm_04_11.c
index 3a2effe..48a87e0 100644
--- a/openbsc/src/libmsc/gsm_04_11.c
+++ b/openbsc/src/libmsc/gsm_04_11.c
@@ -873,7 +873,7 @@
 	struct msgb *msg = gsm411_msgb_alloc();
 	struct gsm_trans *trans;
 	uint8_t *data, *rp_ud_len;
-	uint8_t msg_ref = sms_next_rp_msg_ref(conn);
+	uint8_t msg_ref = sms_next_rp_msg_ref(&conn->next_rp_ref);
 	int transaction_id;
 	int rc;