Fix #14470. Remove w9xpopen per PEP 11.

As stated in PEP 11, 3.4 removes code on Windows platforms where
COMSPEC points to command.com. The w9xpopen project in Visual Studio
was added to support that case, and there was a special case in subprocess
to cover that situation. This change removes the w9xpopen project from
the Visual Studio solution and removes any references to the w9xpopen
executable.