Fix main build (#4446)

src/Analyzers/MSTest.Analyzers/TestContextShouldBeValidAnalyzer.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using System.Collections.Concurrent;
 using System.Collections.Immutable;
 
 using Analyzer.Utilities.Extensions;