Merge pull request #5062 from IvenBach/RemoveUnusedUsingDirective

Minor tech dept; no functionality changes.

Author: retailcoder

Rubberduck.Core/UI/UnitTesting/TestExplorerViewModel.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.ComponentModel;
 using System.Globalization;
 using System.Linq;