src: remove unused misc variable

This commit removes the unused 'misc' variable from one of the
NodeMainInstance constructors.

Another option could be to add a default argument to
SetIsolateMiscHandlers but I'd like to hear what others think about that
first.

PR-URL: https://github.com/nodejs/node/pull/33417


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>