Revert "Provide DLLEXPORT markers for C functions via PG_FUNCTION_INFO_V1 macro."

This reverts commit c8ead2a3.
Seems there is no way to do this that doesn't cause MSVC to give
warnings, so let's just go back to the way we've been doing it.

Discussion: <11843.1478358206@sss.pgh.pa.us>