vscode-extensions.ms-dotnettools.csharp: fix license.

Initially the C# extension was fully open source and had the MIT license, however, after changing to the Roslyn language server and adding the vsdbg binary, it has switched to a non-free license. It even has builtin guards against running on non-microsoft VSCode builds now.
Some things in the C# extension can be built from source, but the debugging layer is fully based on vsdbg which is part of Visual Studio which is proprietary, so the extension as a whole is unfree because of it.