MdeModulePkg PiSmmCore: Remove unreferenced symbol for SMRAM profile

MdeModulePkg PiSmmCore: Remove unreferenced symbol for SMRAM profile

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=370

Use GLOBAL_REMOVE_IF_UNREFERENCED for some SMRAM profile global variables,
then their symbols could be removed when SMRAM profile is disabled.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>