Platform/Styx: get rid of DO_KCS preprocessor macro

Now that all code references the PCD instead, let's drop the macro
from all Styx platforms.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>