src: exclude C++ SetImmediate() from count

There is no real reason to manage a count manually, given that
checking whether there are C++ callbacks is a single pointer
comparison.

This makes it easier to add other kinds of native C++ callbacks
that are managed in a similar way.

PR-URL: https://github.com/nodejs/node/pull/31386
Refs: https://github.com/openjs-foundation/summit/pull/240


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>