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>