ArmPlatformPkg: remove unused ArmPlatformLibNullSec

ArmPlatformLibNullSec is built from a secondary .inf that omits
ArmPlatformLibNullMem.c from the [Sources] section so the library
can be used in a SEC context. This is slightly dodgy, given that
the resulting library is incomplete. Let's just remove this version,
since it isn't used anywhere anyway.

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