Remove process mode training

Refs https://redmine.teklia.com/issues/5980

Depends #1687 (closed)

Once the CreateTrainingProcess endpoint has been removed, we can remove safely the ProcessMode.Training.

A data migration must be created to convert all existing processes with this mode into ProcessMode.Workers , and prefixing their name with (Old Training).

You can also delete the 3 folders Foreign keys on process (train_folder, validation_folder, test_folder).

This will affect only a handful of processes on prod & demo instances