Remove included MSBuild tools and Mono

[JoeRobich]

Background:

The included Mono and MSBuild tools were added to support easy development of C# using .NET Core. We have frequently updated these build tools to match the current shipping SDK tooling.

On Windows we did not need to ship a copy of Mono because we could rely on the .NET Framework being installed. For MacOS and Linux we shipped a minimal Mono that would support running MSBuild and OmniSharp. With these tools, the .NET SDK installed, and the dotnet CLI discoverable from the environment PATH, developers would have good support when developing against the newer SDK-style projects.

There was some support for Full Framework projects with this configuration, but that was considered a bonus. Our most common Full Framework projects (Unity) were unsupported with this configuration because the .NET 4.7.1 targeting pack was not included.

Proposal:

The included MSBuild and Mono should be removed.

We now ship a .NET 6 build of OmniSharp which runs against the build tools in the .NET SDK. The .NET 6 build, with an install of the .NET 6 SDK and the dotnet CLI discoverable from the environment PATH, covers all the scenarios that the included MSBuild Tools and Mono were intended to cover. Users working with Full Framework projects typically install the VS MSBuild Tools or a full Mono to get support for additional project types.

Everyone would benefit from reduced download size and OmniSharp would benefit from the reduced maintenance burden.