src: remove unnecessary req_wrap_obj

The req_wrap_obj is only used in one place which is setting up the
arguments for the MakeCallback call. Removing it to simplify the code
somewhat.

PR-URL: https://github.com/nodejs/node/pull/10942


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>