Revert "src: keep main-thread Isolate attached to platform during Dispose"

This reverts commit e460f8cf.

It is no longer necessary after the previous commit, and restores
consistency of the call order between the main thread code,
the other call sites, and the documentation.

Refs: https://github.com/nodejs/node/pull/31795
PR-URL: https://github.com/nodejs/node/pull/31853


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>