Execution example::

  config_delete nonexistent
  # [
  #   [
  #     -22,
  #     1337566253.89858,
  #     0.000355720520019531,
  #     "[config][delete] failed to delete: <nonexistent>",
  #     [
  #       [
  #         "grn_config_delete",
  #         "lib/config.c",
  #         2929
  #       ]
  #     ]
  #   ],
  #   false
  # ]
