ArmVirtPkg/VirtFdtDxe: remove handling of fw_cfg DT node

ArmVirtPkg/VirtFdtDxe: remove handling of fw_cfg DT node

Remove the handling of the fw_cfg DT node from VirtFdtDxe now that the
fw_cfg client library has been moved to the FDT client protocol, and no
longer relies on VirtFdtDxe to pass this information via dynamic PCDs.
Since the PCDs in question are now no longer used, remove them from the
various DEC and DSC files as well.

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