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_subscriber.c b/src/gprs/gprs_subscriber.c
index aa03509..94297d0 100644
--- a/src/gprs/gprs_subscriber.c
+++ b/src/gprs/gprs_subscriber.c
@@ -22,6 +22,7 @@
 
 #include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
 #include <osmocom/gsm/gsup.h>
+#include <osmocom/gsm/apn.h>
 #include <osmocom/core/utils.h>
 #include <osmocom/core/logging.h>
 #include <openbsc/gprs_subscriber.h>
@@ -371,7 +372,7 @@
 
 		OSMO_ASSERT(pdp_data != NULL);
 		pdp_data->pdp_type = pdp_info->pdp_type;
-		gprs_apn_to_str(pdp_data->apn_str,
+		osmo_apn_to_str(pdp_data->apn_str,
 				pdp_info->apn_enc, pdp_info->apn_enc_len);
 		memcpy(pdp_data->qos_subscribed, pdp_info->qos_enc, pdp_info->qos_enc_len);
 		pdp_data->qos_subscribed_len = pdp_info->qos_enc_len;