ArmPkg: remove unused PcdArmUncachedMemoryMask PCD

This removes the PCD PcdArmUncachedMemoryMask from ArmPkg, along with
any remaining references to it in various platform .DSC files. It is
no longer used now that we removed the virtual uncached pages protocol
and the associated DebugUncachedMemoryAllocationLib library instance.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>