src: remove unused env->vm_parsing_context_symbol

Stopped being used via 77b52fd5, was
originally added in d932e802.

For the one remaining usecase inside of `lib/vm.js`, define a Symbol at
the top of the file.

PR-URL: https://github.com/nodejs/node/pull/22034


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>