Remove unused using

Author: Youssef1313

src/NetAnalyzers/Core/Microsoft.NetCore.Analyzers/Runtime/UseSpanBasedStringConcat.cs

@@ -6,7 +6,6 @@
 using System.Linq;
 using Analyzer.Utilities;
 using Analyzer.Utilities.Extensions;
-using Analyzer.Utilities.PooledObjects;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Operations;