MdeModulePkg: Remove mZeroGuid definition in Universal/Variable/RuntimeDxe

MdeModulePkg: Remove mZeroGuid definition in Universal/Variable/RuntimeDxe

MdeModulePkg has defined gZeroGuid in 'Guid/ZeroGuid.h', therefore, the
mZeroGuid defined in Universal/Variable/RuntimeDxe is redundant.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17840 6f19259b-4bc3-4df7-8a09-765794883524