Migrate from gprs_apn_to_str() to libosmocore osmo_apn_to_str()

In 2015, Jacob moved/copied related functions to libosmocore, but
for some reason didn't remove the copies here.  Let's follow-up on
that and remove duplicated code.

The libosmocore commit introducing osmo_apn_to_str() was
8114294bf29ac6e44822c0ae43d4b0819f11b022

Change-Id: I7315ffcbed8a54cca2056f313bb7783ad82d0ee9
diff --git a/src/gprs/gprs_sgsn.c b/src/gprs/gprs_sgsn.c
index 93b133f..43eeaaa 100644
--- a/src/gprs/gprs_sgsn.c
+++ b/src/gprs/gprs_sgsn.c
@@ -30,6 +30,7 @@
 #include <osmocom/gprs/gprs_ns.h>
 #include <osmocom/gprs/gprs_bssgp.h>
 #include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
+#include <osmocom/gsm/apn.h>
 
 #include <openbsc/gprs_subscriber.h>
 #include <openbsc/debug.h>
@@ -758,7 +759,7 @@
 			return NULL;
 		}
 
-		gprs_apn_to_str(req_apn_str,
+		osmo_apn_to_str(req_apn_str,
 				TLVP_VAL(tp, GSM48_IE_GSM_APN),
 				TLVP_LEN(tp, GSM48_IE_GSM_APN));