Switched MobManager::step() to the timer.

It doesn't really need to run on every shard step. This makes it a lot
less hot.

src/CNShardServer.cpp

@@ -87,6 +87,4 @@ void CNShardServer::onStep() {
             event.scheduledEvent = currTime + event.delta;
         }
     }
-
-    MobManager::step(currTime);
 }