ggsn_vty: Stop using deprecated API vty_install_default

Changes made as requested by the deprecation text.

Fixes warning below:
 warning: ‘vty_install_default’ is deprecated: Now happens implicitly with install_node() [-Wdeprecat
ed-declarations]
  vty_install_default(GGSN_NODE);
  ^~~~~~~~~~~~~~~~~~~

Change-Id: I5c6197129e0c251a4e8dd174027b011c8f6476c6
diff --git a/ggsn/ggsn_vty.c b/ggsn/ggsn_vty.c
index 9f343ec..6e15ae4 100644
--- a/ggsn/ggsn_vty.c
+++ b/ggsn/ggsn_vty.c
@@ -856,8 +856,8 @@
 
 	install_element(CONFIG_NODE, &cfg_ggsn_cmd);
 	install_element(CONFIG_NODE, &cfg_no_ggsn_cmd);
+
 	install_node(&ggsn_node, config_write_ggsn);
-	vty_install_default(GGSN_NODE);
 	install_element(GGSN_NODE, &cfg_description_cmd);
 	install_element(GGSN_NODE, &cfg_no_description_cmd);
 	install_element(GGSN_NODE, &cfg_ggsn_shutdown_cmd);
@@ -872,7 +872,6 @@
 	install_element(GGSN_NODE, &cfg_ggsn_no_default_apn_cmd);
 
 	install_node(&apn_node, NULL);
-	vty_install_default(APN_NODE);
 	install_element(APN_NODE, &cfg_description_cmd);
 	install_element(APN_NODE, &cfg_no_description_cmd);
 	install_element(APN_NODE, &cfg_apn_shutdown_cmd);