Remove unused using

Author: mee-ironsoftware

IronSoftware.Drawing/IronSoftware.Drawing.Common.Tests/UnitTests/ColorFunctionality.cs

@@ -1,7 +1,5 @@
 using FluentAssertions;
-using SixLabors.ImageSharp.PixelFormats;
 using System;
-using System.Runtime.CompilerServices;
 using Xunit;
 using Xunit.Abstractions;