Remove using directive

Author: IvenBach

Rubberduck.Core/UI/CodeExplorer/CodeExplorerWindow.cs

@@ -1,5 +1,4 @@
-using System;
-using System.Diagnostics.CodeAnalysis;
+using System.Diagnostics.CodeAnalysis;
 using System.Windows.Forms;
 using Rubberduck.Navigation.CodeExplorer;
 using Rubberduck.Resources.CodeExplorer;