ARM: decompress: remove unused ARCH_HAS_DECOMP_WDOG

ARCH_HAS_DECOMP_WDOG is only used in lib/inflate.c which is not
included in arch/arm/boot/compressed/decompress.c now.  That said,
ARCH_HAS_DECOMP_WDOG is not used at all.  Let's remove it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>