Removes SageMakerTrainer code but keeps class as wrapper
# What does this PR do?

This PRs removes the `SageMakerTrainer` specific code since it has been merged into Trainer #10975. We had issues and bugs in `SageMakerTrainer` but not in `Trainer` since this class is already deprecated and will be removed in v5 of Transformers I removed the code for the next release. 
`SageMakerTrainer` will still work since all functionality has been migrated to `Trainer`. I tested it with `run_glue.py` 