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/tests/gsm0408/gsm0408_test.c b/openbsc/tests/gsm0408/gsm0408_test.c
index 0c7b5ce..9e7babb 100644
--- a/openbsc/tests/gsm0408/gsm0408_test.c
+++ b/openbsc/tests/gsm0408/gsm0408_test.c
@@ -563,15 +563,15 @@
 	memset(&conn, 0, sizeof(conn));
 	conn.next_rp_ref = 255;
 
-	res = sms_next_rp_msg_ref(&conn);
+	res = sms_next_rp_msg_ref(&conn.next_rp_ref);
 	printf("Allocated reference: %d\n", res);
 	OSMO_ASSERT(res == 255);
 
-	res = sms_next_rp_msg_ref(&conn);
+	res = sms_next_rp_msg_ref(&conn.next_rp_ref);
 	printf("Allocated reference: %d\n", res);
 	OSMO_ASSERT(res == 0);
 
-	res = sms_next_rp_msg_ref(&conn);
+	res = sms_next_rp_msg_ref(&conn.next_rp_ref);
 	printf("Allocated reference: %d\n", res);
 	OSMO_ASSERT(res == 1);
 }