Camlinternalformat: remove faulty check_open_box function
Camlinternalformat: remove faulty check_open_box function

- Currently the check is a no-op, because the "emit a warning if the
  check fails" was never implemented. (It would actually require some
  work to pass a source location there to emit a warning, so it's not
  trivial.)

- The check is implemented by calling `open_box_of_string` and
  catching the `Failure _` exception if that function fails. This is
  just wrong: `Failure _` should be reserved to fatal program errors,
  and should not be caught for control-flow.

- The current implement is buggy (it fails all the time, but we don't
  notice because no warning is emitted):
  CamlinternalFormat.open_box_of_string expects a string of the form
  "v 3", but check_open_box would pass a string of the form "<v 3>"
  (or an empty string), which is the payload of the format value. So
  the check always fails.

- The idea of the check is wrong: "@[<x>foo@]" is an incorrect format
  string to pass to Format (the box indication does not make sense),
  but it is a perfectly fine format string to pass to Printf, where it
  just prints "@[<x>foo@]" on the output. So we cannot complain to the
  user at type-checking time, when we don't know how the format string
  will be used, whether the boxes will be interpreted as actual boxes
  or string literals.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml