ArmVirtPkg/VirtFdtDxe: remove GIC discovery

ArmVirtPkg/VirtFdtDxe: remove GIC discovery

Now that we moved the GIC discovery to our ArmGicArchLib implementation,
we can remove it from VirtFdtDxe, since it is no longer used. Remove the
PcdArmGicRevision declaration and definitions as well: VirtFdtDxe no longer
sets it, and no other drivers consume its value.

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