ARM: SAMSUNG: Delete unused plat/regs-serial.h header file

There are no more users of this file, it can be deleted now.
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>

arch/arm/plat-samsung/include/plat/regs-serial.h

-#include <linux/serial_s3c.h>