UefiCpuPkg: PiSmmCpuDxeSmm: Remove unused references to SmmLib

The PiSmmCpuDxeSmm module does not use any services from the SmmLib.
This change removes the SmmLib from PiSmmCpuDxeSmm module and also
removes the lib mapping in the UefiCpuPkg DSC file because no other
modules in the UefiCpuPkg use the SmmLib.

Removal of SmmLib is now possible because the only API call to it,
ClearSmi(), was ultimately removed from PiSmmCpuDxeSmm -- see the
"BUGBUG" comment in git commit 529a5a86.

Cc: "Yao, Jiewen" <jiewen.yao@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: "Yao, Jiewen" <jiewen.yao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18673 6f19259b-4bc3-4df7-8a09-765794883524