remove unused import (#163)

tutorials/pick_and_place/PickAndPlaceProject/Assets/DemoScripts/Demo.cs

 using RosSharp;
 using RosSharp.Control;
 using RosSharp.Urdf.Editor;
-using Unity.EditorCoroutines.Editor;
 using UnityEditor;
 using UnityEngine;
 using Unity.Robotics.ROSTCPConnector;