Remove duplicate function prototype.

Kyotaro Horiguchi

src/backend/libpq/pqcomm.c

@@ -145,7 +145,6 @@ static void socket_startcopyout(void);
 static void socket_endcopyout(bool errorAbort);
 static int	internal_putbytes(const char *s, size_t len);
 static int	internal_flush(void);
-static void socket_set_nonblocking(bool nonblocking);
 
 #ifdef HAVE_UNIX_SOCKETS
 static int	Lock_AF_UNIX(char *unixSocketDir, char *unixSocketPath);