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

src/server/game/Spells/SpellMgr.cpp

@@ -351,6 +351,12 @@ SpellMgr::~SpellMgr()
     UnloadSpellInfoStore();
 }
 
+SpellMgr* SpellMgr::instance()
+{
+    static SpellMgr instance;
+    return &instance;
+}
+
 /// Some checks for spells, to prevent adding deprecated/broken spells for trainers, spell book, etc
 bool SpellMgr::ComputeIsSpellValid(SpellInfo const *spellInfo, bool msg)
 {