Update AlphaDecoder.cs

Author: JimBobSquarePants

Diff for: src/ImageSharp/Formats/Webp/AlphaDecoder.cs

@@ -6,7 +6,6 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.Intrinsics;
-using System.Runtime.Intrinsics.Arm;
 using System.Runtime.Intrinsics.X86;
 using SixLabors.ImageSharp.Formats.Webp.BitReader;
 using SixLabors.ImageSharp.Formats.Webp.Lossless;