[ospf6] Add no router ospf6

Suggestion: Makes no sense to me that the statement to remove ospf6
configuration is located in OSPF6D_NODE.
Moved to CONFIG_NODE next to matching define command.

ospf6d/ospf6_top.c

@@ -667,6 +667,7 @@ ospf6_top_init (void)
   install_element (VIEW_NODE, &show_ipv6_ospf6_cmd);
   install_element (ENABLE_NODE, &show_ipv6_ospf6_cmd);
   install_element (CONFIG_NODE, &router_ospf6_cmd);
+  install_element (CONFIG_NODE, &no_router_ospf6_cmd);
 
   install_element (VIEW_NODE, &show_ipv6_ospf6_route_cmd);
   install_element (VIEW_NODE, &show_ipv6_ospf6_route_detail_cmd);