src: drop the NODE_ISOLATE_SLOT macro

The `NODE_ISOLATE_SLOT` macro has been unused since c3cd453c.

PR-URL: https://github.com/nodejs/node/pull/11692


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>