Drop use of deprecated vty is_config_node() cb
This callback was drepecated and is not ever called since
libosmocore.git 70ce871532ab21955e0955d7e230eae65438f047 (release 1.3.0).
See also libosmocore.git d31de237582f6fe3315d61bb9a488d4cda92654e.
Change-Id: Ib10b939fbec2620b22c739c57aecbf9d45c27898
