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_vty.c b/src/gprs/gb_proxy_vty.c
index 933b6b0..86d65a8 100644
--- a/src/gprs/gb_proxy_vty.c
+++ b/src/gprs/gb_proxy_vty.c
@@ -29,6 +29,7 @@
 
 #include <openbsc/gsm_04_08.h>
 #include <osmocom/gprs/gprs_ns.h>
+#include <osmocom/gsm/apn.h>
 
 #include <openbsc/debug.h>
 #include <openbsc/gb_proxy.h>
@@ -107,7 +108,7 @@
 	       if (g_cfg->core_apn_size > 0) {
 		       char str[500] = {0};
 		       vty_out(vty, " core-access-point-name %s%s",
-			       gprs_apn_to_str(str, g_cfg->core_apn,
+			       osmo_apn_to_str(str, g_cfg->core_apn,
 						 g_cfg->core_apn_size),
 			       VTY_NEWLINE);
 	       } else {