Remove netstandard2.0 target from Microsoft.CodeAnalysis.Remote.Razor

[chsienki]

Microsoft.CodeAnalysis.Remote.Razor builds for both .NET and netstandard2.0. It appears that the netstandard build isn't actually shipped by anything. We should remove the target to make the build cleaner and faster.

Originally posted by @DustinCampbell in #12385 (comment)

[chsienki]

A thought occurs if we need to netstandard2.0 the compiler bits at all? I think we only ship them in .NET hosts at this point.

We also drop the compiler in the root VSIX (I think for the original re-director logic) but now that we're shipping it as part of the core tooling in ServiceHubCore we should probably drop that.