Remove obsolete TODO (already completed)

Author: drewnoakes

docs/build-acceleration.md

@@ -106,8 +106,6 @@ Looking through the build output with the following points in mind:
 
    Then build acceleration will not know whether it is safe to copy a modified output DLL from a referenced project or not. We rely on the use of reference assemblies to convey this information. To address this, ensure all referenced projects have the `ProduceReferenceAssembly` property set to `true`. You may like to add this to your `Directory.Build.props` file alongside the `AccelerateBuildsInVisualStudio` property. Note that projects targeting `net5.0` or later produce reference assemblies by default. Projects that target .NET Standard may require this to be specified manually (see https://github.com/dotnet/project-system/issues/8865).
 
-- 🗒️ TODO Add validation and output message when reference assemblies are not enabled (https://github.com/dotnet/project-system/issues/8798)
-
 - ✅ You should see a section listing items to copy:
 
    ```