Revert "embedding: make Stop() stop Workers"

This reverts commit 037ac99e.

As flaky CI failures have revealed, this feature was implemented
incorrectly. `stop_sub_worker_contexts()` needs to be called on the
thread on which the `Environment` is currently running, it’s not
thread-safe. The current API requires `Stop()` to be thread-safe,
though.

We could add a new API for this, but unless there’s demand, that’s
probably not necessary as `FreeEnvironment()` will also stop Workers,
which is commonly the next action on an `Environment` instance after
having `Stop()` called on it.

Refs: https://github.com/nodejs/node/pull/32531

PR-URL: https://github.com/nodejs/node/pull/32623


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