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.

src/interfaces/libpq/win32.mak

@@ -16,7 +16,7 @@ CPU=i386
 !MESSAGE Building the Win32 static library...
 !MESSAGE
 !ELSEIF ("$(CPU)" == "IA64")||("$(CPU)" == "AMD64")
-ADD_DEFINES=/D "_WIN64" /Wp64 /GS
+ADD_DEFINES=/Wp64 /GS
 ADD_SECLIB=bufferoverflowU.lib
 !MESSAGE Building the Win64 static library...
 !MESSAGE