UefiCpuPkg/PiSmmCpuDxeSmm: Remove MTRR field from PSD

UefiCpuPkg/PiSmmCpuDxeSmm: Remove MTRR field from PSD

https://bugzilla.tianocore.org/show_bug.cgi?id=277

The MTRR field was removed from PROCESS_SMM_DESCRIPTOR
structure in commit:

https://github.com/tianocore/edk2/commit/26ab5ac3621bdefe96987f8c1512ca79e1bb7ac0

However, the references to the MTRR field in assembly
files were not removed.  Remove the extern reference
to gSmiMtrr and set the Reserved14 field
of PROCESS_SMM_DESCRIPTOR to 0.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>