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/gb_proxy_patch.c b/src/gprs/gb_proxy_patch.c
index 7bddc44..210fb2b 100644
--- a/src/gprs/gb_proxy_patch.c
+++ b/src/gprs/gb_proxy_patch.c
@@ -28,6 +28,7 @@
 
 #include <osmocom/gprs/protocol/gsm_08_18.h>
 #include <osmocom/core/rate_ctr.h>
+#include <osmocom/gsm/apn.h>
 
 /* patch RA identifier in place */
 static void gbproxy_patch_raid(uint8_t *raid_enc, struct gbproxy_peer *peer,
@@ -101,7 +102,7 @@
 		LOGP(DGPRS, LOGL_DEBUG,
 		     "Patching %s to SGSN: Removing APN '%s'\n",
 		     log_text,
-		     gprs_apn_to_str(str1, apn, apn_len));
+		     osmo_apn_to_str(str1, apn, apn_len));
 
 		*new_apn_ie_len = 0;
 		gprs_msgb_resize_area(msg, apn_ie, apn_ie_len, 0);
@@ -116,8 +117,8 @@
 		     "Patching %s to SGSN: "
 		     "Replacing APN '%s' -> '%s'\n",
 		     log_text,
-		     gprs_apn_to_str(str1, apn, apn_len),
-		     gprs_apn_to_str(str2, peer->cfg->core_apn,
+		     osmo_apn_to_str(str1, apn, apn_len),
+		     osmo_apn_to_str(str2, peer->cfg->core_apn,
 				       peer->cfg->core_apn_size));
 
 		*new_apn_ie_len = peer->cfg->core_apn_size + 2;