OvmfPkg: remove USE_OLD_BDS build fallback macro

OvmfPkg: remove USE_OLD_BDS build fallback macro

Reasons:

- USE_OLD_BDS requires duplicating updates between OVMF's library
  instances that depend on USE_OLD_BDS being FALSE vs. TRUE. Examples:

  d5aee61bfaaa OvmfPkg/QemuNewBootOrderLib: adapt Q35 SATA PMPN to UEFI
               spec Mantis 1353

  1da761664949 OvmfPkg/QemuBootOrderLib: adapt Q35 SATA PMPN to UEFI spec
               Mantis 1353

- The Xen community has embraced the new BDS. Examples:

  14b2ebc30c8b OvmfPkg/PlatformBootManagerLib: Postpone the shell
               registration

  49effaf26ec9 OvmfPkg/PciHostBridgeLib: Scan for root bridges when
               running over Xen

- OVMF doesn't build with "-D USE_OLD_BDS -D HTTP_BOOT_ENABLE" anyway, as
  NetworkPkg/HttpBootDxe now requires UefiBootManagerLib:

  50a65824c74a NetworkPkg: Use UefiBootManagerLib API to create load
               option.

  We (correctly) don't resolve UefiBootManagerLib when USE_OLD_BDS is
  TRUE.

- The new BDS has been working well; for example it's the only BDS
  available in ArmVirtPkg:

  1946faa710e6 ArmVirtPkg/ArmVirtQemu: use MdeModulePkg/BDS

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@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>