MdeModulePkg: Remove PcdIdentifyMappingPageTablePtr

PcdIdentifyMappingPageTablePtr was used to share page
table buffer between modules.
Buf after some changes on 2015/07/17, it was useless
and could be removed.
https://bugzilla.tianocore.org/show_bug.cgi?id=1304

v2:
1.Remove PcdIdentifyMappingPageTablePtr in MdeModulePkg.uni.
2.Update the commit message.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: shenglei <shenglei.zhang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>