ArmPkg/ArmMmuLib: remove VirtualMask arg from ArmSetMemoryAttributes

We no longer make use of the ArmMmuLib 'feature' to create aliased
memory ranges with mismatched attributes, and in fact, it was only
wired up in the ARM version to begin with.

So remove the VirtualMask argument from ArmSetMemoryAttributes()'s
prototype, and remove the dead code that referred to it.

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