Remove docker xunit tests

Author: sblom

test/json-ld.net.tests/DockerTests.cs

@@ -25,7 +25,6 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
-    <PackageReference Include="Xunit.SkippableFact" Version="1.4.13" />
     <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />
   </ItemGroup>