Fix build on .NET core 2.1

src/ImageSharp/Metadata/Profiles/XMP/XElementEqualityComparer.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Xml.Linq;
 
 namespace SixLabors.ImageSharp.Metadata.Profiles.Xmp
@@ -13,9 +12,9 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Xmp
     public class XElementEqualityComparer : IEqualityComparer<XElement>
     {
         /// <inheritdoc />
-        public bool Equals([AllowNull] XElement x, [AllowNull] XElement y) => x.Name == y.Name && x.Value == y.Value;
+        public bool Equals(XElement x, XElement y) => x.Name == y.Name && x.Value == y.Value;
 
         /// <inheritdoc />
-        public int GetHashCode([DisallowNull] XElement obj) => obj.GetHashCode();
+        public int GetHashCode(XElement obj) => obj.GetHashCode();
     }
 }

tests/ImageSharp.Tests/Metadata/Profiles/XMP/XmpProfileTests.cs

@@ -19,15 +19,15 @@ namespace SixLabors.ImageSharp.Tests.Metadata.Profiles.Xmp
 {
     public class XmpProfileTests
     {
-        private static GifDecoder GifDecoder => new GifDecoder() { IgnoreMetadata = false };
+        private static GifDecoder GifDecoder => new() { IgnoreMetadata = false };
 
-        private static JpegDecoder JpegDecoder => new JpegDecoder() { IgnoreMetadata = false };
+        private static JpegDecoder JpegDecoder => new() { IgnoreMetadata = false };
 
-        private static PngDecoder PngDecoder => new PngDecoder() { IgnoreMetadata = false };
+        private static PngDecoder PngDecoder => new() { IgnoreMetadata = false };
 
-        private static TiffDecoder TiffDecoder => new TiffDecoder() { IgnoreMetadata = false };
+        private static TiffDecoder TiffDecoder => new() { IgnoreMetadata = false };
 
-        private static WebpDecoder WebpDecoder => new WebpDecoder() { IgnoreMetadata = false };
+        private static WebpDecoder WebpDecoder => new() { IgnoreMetadata = false };
 
         [Theory]
         [WithFile(TestImages.Gif.Receipt, PixelTypes.Rgba32)]