bpo-36763: Remove _PyCoreConfig.dll_path

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https://bugs.python.org/issue36763

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According to @zooba, there is no benefit to configure dll_path:

The dll_path field of PyConfig says "Windows only". Does that meant the
struct doesn't have that field except in a Windows build? Or is it
ignored, instead? If it doesn't have that field at all, what #define can
be used to determine if the PyConfig struct will have it or not?

This field doesn't need to be here. It exists because it was used in
getpathp.c, and Victor's internal refactoring has kept it around through
all the field movement.

https://mail.python.org/pipermail/python-dev/2019-May/157451.html

I understood that it would allow to decide where Python looks for DLL. It seems like I misunderstood the purpose of this field. So let's remove it :-)