Fix macro problem with gai_strerror on Windows.

Commit 5579388d was confused about why gai_strerror() didn't work, and
used gai_strerrorA().  It turns out that we had explicitly undefined
Windows' own macro for that somewhere else.  Get rid of all that, and
use the system headers' definition of gai_sterror() directly as
intended.

Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

src/include/port/win32/netdb.h

@@ -4,6 +4,4 @@
 
 #include <ws2tcpip.h>
 
-#define gai_strerror gai_strerrorA
-
 #endif

src/include/port/win32/sys/socket.h

@@ -23,11 +23,4 @@
 #define ERROR PGERROR
 #endif
 
-/*
- * we can't use the windows gai_strerror{AW} functions because
- * they are defined inline in the MS header files. So we'll use our
- * own
- */
-#undef gai_strerror
-
 #endif							/* WIN32_SYS_SOCKET_H */