ArmPkg/ArmLib: remove unused ArmCleanDataCacheToPoU()

ArmPkg/ArmLib: remove unused ArmCleanDataCacheToPoU()

The function ArmCleanDataCacheToPoU() has no users, and its purpose
is unclear, since it uses cache maintenance by set/way to perform
the clean to PoU, which is a dubious practice to begin with. So
remove the declaration and all definitions.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18752 6f19259b-4bc3-4df7-8a09-765794883524