serial_exit: punt unused prototype

No code defines or calls this, so drop the prototype.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Acked-by: NSimon Glass <sjg@chromium.org>

include/common.h

@@ -518,7 +518,6 @@ void ft_pci_setup(void *blob, bd_t *bd);
 
 /* $(CPU)/serial.c */
 int	serial_init   (void);
-void	serial_exit   (void);
 void	serial_setbrg (void);
 void	serial_putc   (const char);
 void	serial_putc_raw(const char);