OvmfPkg/PciHostBridgeLibScan: remove QEMU (fw_cfg) support

OvmfPkg/PciHostBridgeLibScan: remove QEMU (fw_cfg) support

The "OvmfPkg/Library/PciHostBridgeLibScan/PciHostBridgeLibScan.inf"
instance is used in the following platforms in edk2:

  OvmfPkg/Bhyve/BhyveX64.dsc
  OvmfPkg/OvmfXen.dsc

Both platforms define "PcdPciDisableBusEnumeration" with Fixed-at-Build
access method, and TRUE value. Remove the PCD from the
PciHostBridgeLibScan instance, and everything else that is useful only
when the PCD is FALSE.

In practice, this removes the PciHostBridgeUtilityGetRootBridges()
function call, which is based on fw-cfg; see
"OvmfPkg/Library/PciHostBridgeUtilityLib/PciHostBridgeUtilityLib.c".

(Note that the dependency on PciHostBridgeUtilityLib remains in place,
given that the PciHostBridgeLibScan instance continues using lower-level
functions from the library that do not depend on fw-cfg.)

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Peter Grehan <grehan@freebsd.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-34-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>