remove unused using.

src/Compilers/Core/Portable/Text/TextLine.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
-using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Text
@@ -199,4 +198,4 @@ public override int GetHashCode()
             return Hash.Combine(_text, Hash.Combine(_start, _endIncludingBreaks));
         }
     }
-}
+}
\ No newline at end of file