src: remove inspector main_thread_request_ field

This is redundant to the platform notification mechanism, and
the handle may not be cleaned up util we attempt to close the loop.

Refs: https://github.com/nodejs/node/pull/26089
Refs: https://github.com/nodejs/node/pull/26006

PR-URL: https://github.com/nodejs/node/pull/26137


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>