Redundant sep_token_extra option for RoBERTa Fine-tuning
## 🐛 Bug

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Model I am using (Bert, XLNet....): RoBERTa

Language I am using the model on (English, Chinese....): English

## Context

I was reading the code on RoBERTa fine-tuning and noticed the [`sep_token_extra` option](https://github.com/huggingface/pytorch-transformers/search?q=sep_token_extra&unscoped_q=sep_token_extra), which looks like a misinterpretation of a sentence from the original paper.

The current implementation [added an extra `[SEP]` to the RoBERTa input compared to BERT](https://github.com/huggingface/pytorch-transformers/blob/d8923270e6c497862f990a3c72e40cc1ddd01d4e/examples/utils_glue.py#L453), which seems wrong. Check out:

1. The [Facebook language model format](https://github.com/pytorch/fairseq/blob/e75cff5f2c1d62f12dc911e0bf420025eb1a4e33/fairseq/data/legacy/masked_lm_dataset.py#L193)
2. A related [Twitter discussion](https://twitter.com/VictoriaLinML/status/1175596109009321986)

The tasks I am working on is:
Fine-tuning RoBERTa on downstream tasks

## Code Sample
https://github.com/huggingface/pytorch-transformers/search?q=sep_token_extra&unscoped_q=sep_token_extra
Myle Ott from Facebook commented on the Twitter thread (https://twitter.com/myleott/status/1175750596630056961) confirming that there is an extra separator being used, so there should be details I did not understand well.

I will revisit this issue when I understand it better.<|||||>The `sep_token_extra` param is deprecated, as we have simpler ways to do this now (thanks to @LysandreJik). Closing this for now, feel free to re-open if needed.