ocaml-json: drop redundant dune (modules ...) and unused fmt dep

Dune auto-discovers all .ml/.mli files in the library directory; the
explicit (modules ...) list added nothing. The fmt dep was added
earlier but is unused: json.ml does "module Fmt = Core.Fmt" and uses
the internal mini-Fmt in core.ml (Buenzli's design — avoids pulling
in fmt).