[mdoc] Convert to SDK style projects (#642)

Converts `mdoc.csproj` and `Monodoc.Test.csproj` files to SDK style
projects to allow them to be built with dotnet build.

Author: pjcollins

mdoc/Mono.Documentation/Updater/Formatters/CppFormatters/CppWinRtFullMemberFormatter.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Runtime.Remoting.Contexts;
 using System.Text;
 using Mono.Cecil;