OvmfPkg: Drop build flag USE_LEGACY_ISA_STACK and legacy ISA stack

OvmfPkg: Drop build flag USE_LEGACY_ISA_STACK and legacy ISA stack

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1842

According to the discussion at:
https://edk2.groups.io/g/devel/topic/30918343#38093

For OVMF, we keep both ISA stacks:
* The legacy one in PcAtChipsetPkg/IntelFrameworkModulePkg
* The Sio bus based OVMF-specified one introduced by commit a5cc178aeb

for a period of time (includes 1 stable tag: edk2-stable201905). And we
also keep the Sio bus based OVMF-specified stack as the default one (via a
build option 'USE_LEGACY_ISA_STACK') to validate its stability.

This commit will propose to drop the legacy ISA stack from OVMF and remove
the usage of the build flag 'USE_LEGACY_ISA_STACK' at the same time. This
is considered as a preparation for the removal of
PcAtChipsetPkg/IsaAcpiDxe & IntelFrameworkModulePkg.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>