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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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