OvmfPkg: remove USE_OLD_PCI_HOST build option

OvmfPkg: remove USE_OLD_PCI_HOST build option

It's been a month since the following commits appeared in the repo:

  4014885ffdfa8 OvmfPkg: switch to MdeModulePkg/Bus/Pci/PciHostBridgeDxe
  c47ed6fcb5e60 OvmfPkg: match PCI config access to machine type (if not
                USE_OLD_PCI_HOST)

in which we introduced the USE_OLD_PCI_HOST fallback, and made other work
depend on it. I have not heard of any problems (primarily from the
vfio-users group that uses Gerd's daily / hourly OVMF builds), so it's
time to drop the fallback.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>