Cleanup more code and comments related to Windows NT4 (XP days)

All the code and comments cleaned up here is irrelevant since 495ed0e.
Note that this removes an assumption that CreateRestrictedToken() may
not exist, something that could have happened when running under Windows
NT as the code stated.  Rather than assuming that it may not exist, this
causes pg_ctl to fail hard if the function cannot be loaded.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20220826112637.GD2342@telsasoft.com

src/backend/port/win32/socket.c

@@ -495,7 +495,7 @@ pgwin32_send(SOCKET s, const void *buf, int len, int flags)
 			return -1;
 		}
 
-		/* No error, zero bytes (win2000+) or error+WSAEWOULDBLOCK (<=nt4) */
+		/* No error, zero bytes */
 
 		if (pgwin32_waitforsinglesocket(s, FD_WRITE | FD_CLOSE, INFINITE) == 0)
 			return -1;