gh-121381 Remove subprocess._USE_VFORK escape hatch

[cmaloney]

This flag was added as an escape hatch in bpo-47245 and backported to Python 3.10. The flag broke at some point between its addition and now. As there is currently no publicly known environments that require this, remove it rather than work on fixing it.

This leaves the flag in the subprocess module to not break code which may have used / checked the flag itself.

discussion: https://discuss.python.org/t/subprocess-use-vfork-escape-hatch-broken-fix-or-remove/56915/2

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📚 Documentation preview 📚: https://cpython-previews--121383.org.readthedocs.build/

• Issue: gh-121381