serial_exit: punt unused prototype

No code defines or calls this, so drop the prototype.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/include/common.h b/include/common.h
index eb19a44..a55600b 100644
--- a/include/common.h
+++ b/include/common.h
@@ -518,7 +518,6 @@
 
 /* $(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);