ArmVirtPkg/VirtFdtDxe: drop PCI host bridge handling

ArmVirtPkg/VirtFdtDxe: drop PCI host bridge handling

Now that the PCI host bridge driver parses the DT node that describes
the PCI host bridge directly via the FDT client protocol, we can drop the
handling from VirtFdtDxe completely.

This means some PCI related PCDs are no longer set, such as PcdPciBusMin,
PcdPciBusMax, PcdPciIoBase, PcdPciIoSize, PcdPciIoTranslation,
PcdPciMmio32Base and PcdPciMmio32Size. Since these PCDs are specific to
ARM (and declared in ArmPlatformPkg), and not used anywhere else by the
ArmVirtPkg platforms, we can simply stop populating them, and drop all
references to them.

It also means that we can no longer rely on PcdPciDisableBusEnumeration
to be set before it is consumed by PciBusDxe and QemuFwCfgAcpiPlatformDxe,
so make those depend on FdtPciPcdProducerLib explicitly via NULL library
class resolution.

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