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>