remove reference to UIElements

Tests/Editor/CanvasAndLayers.cs

 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.ui;
 using UnityEditor;
-using UnityEditor.Experimental.UIElements;
 using UnityEngine;
 using Color = Unity.UIWidgets.ui.Color;
 using Gradient = Unity.UIWidgets.ui.Gradient;
             this._selected = 0;
 
             this.titleContent = new GUIContent("CanvasAndLayers");
-            this.SetAntiAliasing(4);
         }
 
         void OnGUI() {