[Clean up] Datasets
Clean up `nlp/datasets` folder. 

As I understood, eventually the `nlp/datasets` shall not exist anymore at all. 

The folder `nlp/datasets/nlp` is kept for the moment, but won't be needed in the future, since it will live on S3 (actually it already does) at: `https://s3.console.aws.amazon.com/s3/buckets/datasets.huggingface.co/nlp/?region=us-east-1` and the different `dataset downloader scripts will be added to `nlp/src/nlp` when downloaded by the user. 

The folder `nlp/datasets/checksums` is kept for now, but won't be needed anymore in the future. 

The remaining folders/ files are leftovers from tensorflow-datasets and are not needed. The can be looked up in the private tensorflow-dataset repo.
Yes!