feat(Core/Misc): remove and replace ACE_Singleton (#2418)

src/server/game/Battlefield/BattlefieldMgr.h

@@ -8,7 +8,6 @@
 #define BATTLEFIELD_MGR_H_
 
 #include "Battlefield.h"
-#include "ace/Singleton.h"
 
 class Player;
 class GameObject;
@@ -25,6 +24,8 @@ class BattlefieldMgr
     // dtor
     ~BattlefieldMgr();
 
+    static BattlefieldMgr* instance();
+
     // create battlefield events
     void InitBattlefield();
     // called when a player enters an battlefield area
@@ -62,6 +63,6 @@ class BattlefieldMgr
     uint32 m_UpdateTimer;
 };
 
-#define sBattlefieldMgr ACE_Singleton<BattlefieldMgr, ACE_Null_Mutex>::instance()
+#define sBattlefieldMgr BattlefieldMgr::instance()
 
 #endif