Platform, Silicon: drop gEmbeddedTokenSpaceGuid.PcdPrePiCpuMemorySize

gEmbeddedTokenSpaceGuid.PcdPrePiCpuMemorySize will be removed, so
drop any overrides from the platforms in edk2-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>