Avoid duplicate Microsoft.CodeAnalysis DLLs

[jjonescz]

With the dotnet run app.cs feature, we now deploy Microsoft.CodeAnalysis.CSharp.dll next to dotnet.dll in the SDK. But that seems unnecessary since we have another copy of that in Roslyn/bincore. We could remove the DLLs in the root folder and make dotnet.dll load the ones from Roslyn/bincore. Perhaps through probing paths in deps.json.