removes Roslynator.Testing.CSharp.Xunit (#23)

I made a bad assumption, `Roslynator.Testing.CSharp.Xunit` is not relevant

Author: rheone

src/NetDuid.Tests/NetDuid.Tests.csproj

@@ -86,10 +86,6 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="Roslynator.Testing.CSharp.Xunit" Version="4.13.1">
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-      <PrivateAssets>all</PrivateAssets>
-    </PackageReference>
     <PackageReference Include="SonarAnalyzer.CSharp" Version="10.7.0.110445">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>