api: config: stop using _make_config_values
api: config: stop using _make_config_values

Now that named_value::value_as_json() exists, make use of it to report the
current value of a configuration variable via the REST API, instead of
_make_config_values().

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla