代码清理

Ink Canvas/MainWindow_cs/MW_Eraser.cs

@@ -1,4 +1,5 @@
-using System;
+using Ink_Canvas.Helpers;
+using System;
 using System.Diagnostics;
 using System.Linq;
 using System.Windows;
@@ -6,7 +7,6 @@ using System.Windows.Controls;
 using System.Windows.Ink;
 using System.Windows.Input;
 using System.Windows.Media;
-using Ink_Canvas.Helpers;
 
 namespace Ink_Canvas
 {