Removed unused usings

src/ImageSharp/Formats/Jpeg/Components/Encoder/HuffmanScanEncoder.cs

@@ -1,9 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using System.IO;
-using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using SixLabors.ImageSharp.Memory;

src/ImageSharp/Formats/Jpeg/Components/Encoder/YCbCrForwardConverter444{TPixel}.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.PixelFormats;