remove net46 from tests

tests/Microsoft.CodeAnalysis.Testing/Directory.Build.props

@@ -21,7 +21,7 @@
   <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
 
   <PropertyGroup>
-    <TestTargetFrameworks>netcoreapp3.1;net472;net46</TestTargetFrameworks>
+    <TestTargetFrameworks>netcoreapp3.1;net472</TestTargetFrameworks>
     <SourceGeneratorTestTargetFrameworks>netcoreapp3.1;net472</SourceGeneratorTestTargetFrameworks>
 
     <!-- Workaround dependencies that do not yet support netcoreapp3.1 https://github.com/dotnet/roslyn/issues/45114 -->