Core/Game: Move scheduled map scripts counter methods into MapManager

* Since those have nothing to do with core scripts and are
  not wished inside the ScriptMgr

(cherry picked from commit 59e3cf82ac)

src/server/game/Scripting/ScriptMgr.cpp

@@ -241,7 +241,7 @@ struct TSpellSummary
 } *SpellSummary;
 
 ScriptMgr::ScriptMgr()
-  : _scriptCount(0), _scheduledScripts(0), _script_loader_callback(nullptr)
+  : _scriptCount(0), _script_loader_callback(nullptr)
 {
 }