The config header parsing code was supposed to be only invoked when
cross-compiling for Windows, but in reality it fails to correctly parse
the config header files shipped with the upstream Python for Windows.

Given that there are now better options for reliable cross-compiling
for Windows such as `PYO3_CROSS_PYTHON_VERSION` or the `abi3-py3*` features,
it should be OK to remove this config for v0.14.

Update the cross-compilation instructions section of the user guide.

Fixes PyO3#1337