sgsn: Fix the unimplemented/uninstalled show ggsn command

This is fixing a GCC and Coverity warning.

GCC:
sgsn_vty.c: At top level:
sgsn_vty.c:308:1: warning: ‘show_ggsn_cmd’ defined but not used [-Wunused-variable]

Coverity: CID 1040727
diff --git a/openbsc/src/gprs/sgsn_vty.c b/openbsc/src/gprs/sgsn_vty.c
index 848fa83..9865fde 100644
--- a/openbsc/src/gprs/sgsn_vty.c
+++ b/openbsc/src/gprs/sgsn_vty.c
@@ -305,13 +305,6 @@
 	return CMD_SUCCESS;
 }
 
-DEFUN(show_ggsn, show_ggsn_cmd,
-	"show ggsn",
-	"")
-{
-
-}
-
 DEFUN(show_pdpctx_all, show_pdpctx_all_cmd,
 	"show pdp-context all",
 	SHOW_STR "Display information on PDP Context\n" "Show everything\n")