ArmVirtPkg/PrePi: remove bogus primary core check

QEMU and KVM based ARM/AARCH64 virtual machines only enter UEFI on
a single core, so ArmPlatformIsPrimaryCore() always returns true.
And even if it didn't, our code does absolutely nothing meaningful
based on its return value, so don't bother calling it, and remove
another frivolous dependency on ArmPlatformLib.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>