remove unused using

com.unity.uiwidgets/Runtime/rendering/proxy_box.cs

 using Unity.UIWidgets.gestures;
 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.ui;
-using UnityEngine;
 using Color = Unity.UIWidgets.ui.Color;
 using Gradient = Unity.UIWidgets.ui.Gradient;
 using Rect = Unity.UIWidgets.ui.Rect;