The _posixsubprocess module is now required on POSIX.

The _posixsubprocess module is now required on POSIX.
Remove the pure Python POSIX subprocess implementation.

If non-CPython VMs (are there any for 3.x yet?) were somehow depending
on this, they already have the exact same set of problems with Python
code being executed after os.fork() that _posixsubprocess was written
to deal with.  They should implement an equivalent outside of Python.