ArmVirtPkg: remove ArmCpuLib references

ArmCpuLib is never used anywhere, and is about to be removed. So remove
any references from our .DSC files.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>