ARM: samsung: remove unused arch_decomp_wdog() code

Besides the fact that CONFIG_S3C_BOOT_WATCHDOG is defined nowhere, with
ARCH_HAS_DECOMP_WDOG removed from arch/arm/boot/compressed/decompress.c,
all the arch_decomp_wdog() related code is not used.  Remove them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>