Silicon/AMD: drop ArmBds remnant Pcd imports from StyxDtbLoaderLib

ArmPkg PcdArmLinuxFdtMaxOffset and PcdArmLinuxFdtAlignment are still
referenced in StyxDtbLoaderLib.inf, but not actually used anywhere -
so drop them.

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