src: remove node::InitializeV8Platform()

This API method was introduced in commit 90ae4bd0 ("src: add
InitializeV8Platform function") from July 2018 but wasn't properly
exported and therefore not usable on Windows or with shared library
builds.

The motivation from the commit log is mainly about making it easier
to wire up the cctests and there are better ways to do that.

Refs: https://github.com/nodejs/node/pull/31217

PR-URL: https://github.com/nodejs/node/pull/31245


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>