ArmPlatformPkg/PrePi: remove global variable allocation from lowlevel init

ArmPlatformPkg/PrePi: remove global variable allocation from lowlevel init

Now that we dropped all ArmPlatformGlobalVariableLib dependencies,
there is no longer a need to allocate and clear out the global
variable region in the PrePi init code. So remove 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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18992 6f19259b-4bc3-4df7-8a09-765794883524