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/Maps/MapManager.cpp

@@ -38,10 +38,10 @@
 #include "AchievementMgr.h"
 
 MapManager::MapManager()
+    : _nextInstanceId(0), _scheduledScripts(0)
 {
     i_gridCleanUpDelay = sWorld->getIntConfig(CONFIG_INTERVAL_GRIDCLEAN);
     i_timer.SetInterval(sWorld->getIntConfig(CONFIG_INTERVAL_MAPUPDATE));
-    _nextInstanceId = 0;
 }
 
 MapManager::~MapManager() { }