gh-121381 Remove subprocess._USE_VFORK escape hatch (#121383)

gh-121381 Remove subprocess._USE_VFORK escape hatch (#121383)

This flag was added as an escape hatch in gh-91401 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