serial: remove nulldev_serial_input

nulldev_serial_input is static and not used in this file,
so remove it.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/serial_nulldev.c b/drivers/serial/serial_nulldev.c
index efae14d..f3ca7f5 100644
--- a/drivers/serial/serial_nulldev.c
+++ b/drivers/serial/serial_nulldev.c
@@ -22,11 +22,6 @@
 	return 0;
 }
 
-static int nulldev_serial_input(struct udevice *dev)
-{
-	return 0;
-}
-
 static int nulldev_serial_putc(struct udevice *dev, const char ch)
 {
 	return 0;