Remove unused using directive

Author: IvenBach

Rubberduck.Core/UI/CodeExplorer/Commands/ExportCommand.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Windows.Forms;
 using Rubberduck.Interaction;
 using Rubberduck.Navigation.CodeExplorer;