Revert langversion usage

src/Avalonia.Visuals/Avalonia.Visuals.csproj

@@ -3,7 +3,6 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>Avalonia</RootNamespace>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <LangVersion>8</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <EmbeddedResource Include="Assets\*.trie" />