ArmVirtPkg: get rid of A PRIORI DXE declarations for VirtFdtDxe

ArmVirtPkg: get rid of A PRIORI DXE declarations for VirtFdtDxe

We no longer care when VirtFdtDxe executes, since
- the driver sets no dynamic PCDs any longer, and
- the only remaining functionality centers on VirtioMmioInstallDevice()
  and XenIoMmioInstall() function calls and FDT configuration table
  installation.

So drop the A PRIORI declaration.

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