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>

MdeModulePkg/Library/UefiBootManagerLib/BmHotkey.c

@@ -1,7 +1,7 @@
 /** @file
   Hotkey library functions.
 
-Copyright (c) 2011 - 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2011 - 2017, Intel Corporation. All rights reserved.<BR>
 (C) Copyright 2016 Hewlett Packard Enterprise Development LP<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
@@ -220,21 +220,6 @@ BmGetKeyOptions (
   return Param.KeyOptions;
 }
 
-/**
-  Callback function for event.
-  
-  @param    Event          Event for this callback function.
-  @param    Context        Context pass to this function.
-**/
-VOID
-EFIAPI
-BmEmptyFunction (
-  IN EFI_EVENT                Event,
-  IN VOID                     *Context
-  )
-{
-}
-
 /**
   Check whether the bit is set in the value.
 
@@ -901,7 +886,7 @@ EfiBootManagerStartHotkeyService (
   Status = gBS->CreateEvent (
                   EVT_NOTIFY_WAIT,
                   TPL_CALLBACK,
-                  BmEmptyFunction,
+                  EfiEventEmptyFunction,
                   NULL,
                   &mBmHotkeyTriggered
                   );