Remove unnecessary definition of _WIN64 in libpq/win32.mak.

In commit b0e40d18, I should have just
removed the /D switch defining WIN64.  The reason the code worked before
is that all Windows64 compilers automatically predefine _WIN64.  Perhaps
at one time we had code that depended on WIN64 being defined, but it's
long gone, and we should not encourage any reappearance.  Per discussion
with Christian Ullrich.