Remove duplicate training process attributes

In preparation for Dataset processes replacing Training processes, we can remove the attributes on Process that are specific to the Training mode. Instead of using the Process.model_version_id, Process.worker_configuration_id and Process.worker_version_id custom attributes, starting a training process will create a WorkerRun directly for that training worker.

This still leaves the model_id, train_folder_id, test_folder_id and validation_folder_id attributes, which cannot easily be stored elsewhere.

This must not cause changes to the request payload for CreateTrainingProcess, and no changes to the frontend since the frontend does not use those attributes after starting a process.