Remove Microsoft.ML.GenAI.* packages and their supporting code #7720

The Microsoft.ML.GenAI.* packages introduce model-specific implementations, integrations and dependencies that require maintenance alongside the rest of ML.NET.
Removing these packages would reduce the maintenance and dependency surface while preserving the independently useful TorchSharp trainers and tokenizer packages.

Remove the following packages from the repository:

Microsoft.ML.GenAI.Core
Microsoft.ML.GenAI.LLaMA
Microsoft.ML.GenAI.Mistral
Microsoft.ML.GenAI.Phi
Include everything used exclusively to support them:

Associated test projects, approval baselines, model configurations and samples.
Solution entries and any GenAI-specific build or packaging configuration.
GenAI-only NuGet dependencies and unused version properties.
Dedicated design documentation and obsolete third-party notices.
References in current documentation, dependency diagrams and repository tooling guidance.