MdeModulePkg: Use EfiEventEmptyFunction from UefiLib

MdeModulePkg: Use EfiEventEmptyFunction from UefiLib

Use EfiEventEmptyFunction from UefiLib and remove the duplication
of event empty function.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=298

Cc: Feng Tian <feng.tian@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>