APICompat got re-implemented on top of Microsoft.CodeAnalysis and lives
in dotnet/sdk. It also became a shipping product with .NET 6.

The source code was kept around for some time so that consumers could
move to the new tooling. Now seems the right time to finally delete the
code-base.

This tool was already excluded in source build so the deletion won't
result in a build break. If there are still some repos that depend on
the live version of APICompat v1, they will just not get a new
dependency update for it. The code base has been frozen anyway so
consumers shouldn't be impacted by this.