Instead of a json-serialize-use-plists global, use function args

Instead of a json-serialize-use-plists global, use function args

* src/json.c (syms_of_json): Don't defive
json-serialize-use-plists
(Fjson_insert, lisp_to_json, list_to_json_toplevel_1)
(list_to_json_toplevel, Fjson_serialize): Use and pass around
use_plists arg.

* doc/lispref/text.texi:
(Parsing JSON): json-serialize and json-insert now have a
USE-PLISTS arg.

* test/src/json-tests.el (json-serialize/object): Use
json-serialize's USE-PLISTS arg.