OvmfPkg: drop PcdPciDisableBusEnumeration from the IA32, IA32X64, X64 DSCs

OvmfPkg: drop PcdPciDisableBusEnumeration from the IA32, IA32X64, X64 DSCs

With the Xen-dependent PcdSetBoolS() call removed from
OvmfPkg/PlatformPei, the "OvmfPkgIa32.dsc", "OvmfPkgIa32X64.dsc",
"OvmfPkgX64.dsc" platforms never write "PcdPciDisableBusEnumeration". This
means we don't need a dynamic default for the PCD in the DSC files; it
could be declared Fixed-at-Build.

However, because the PCD's default value in "MdeModulePkg.dec" is FALSE,
remove the (same-value) platform defaults altogether.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-23-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>