Remove subclass for sortish sampler
# What does this PR do?

When putting the sortish sampler in the main `Trainer`, I forgot to remove the overide in `Seq2SeqTrainer` which led to an issue (see #9900). This in turn makes the old `finetune_trainer` script fail because its datasets don't have the right entries (the texts are processed during the data collation) so it requires reverting the changes in that script to use back the old `Seq2SeqTrainer`  (which is fine since that script will soon go to legacy).

Fixes #9900