NER: fix construction of input examples for RoBERTa
Hi,

this PR avoids adding an extra `</s>` symbol, so that the final sequence ends with `</s> </s>`.

The documentation clearly states, that ther's only one `</s>` expected at the end of a single sequence:

https://huggingface.co/transformers/model_doc/roberta.html#transformers.RobertaTokenizer.build_inputs_with_special_tokens

The `fairseq` reference implementation shows also only one `</s>` at the end:

https://github.com/pytorch/fairseq/tree/master/examples/roberta#apply-byte-pair-encoding-bpe-to-input-text

Technically, the `sep_token_extra` argument is set to `False` now - I didn't remove this parameter, so future models/tokenizers can use it (when they really need it).

This fixes #4755.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=h1) Report
> Merging [#4943](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/473808da0d476792070f0e7dfebcf1121a12a34f&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4943/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4943      +/-   ##
==========================================
+ Coverage   77.14%   77.20%   +0.06%     
==========================================
  Files         128      128              
  Lines       21745    21745              
==========================================
+ Hits        16775    16789      +14     
+ Misses       4970     4956      -14     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4943/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.42% <0.00%> (ø)` | |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/4943/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `78.65% <0.00%> (+2.29%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=footer). Last update [473808d...e9eb626](https://codecov.io/gh/huggingface/transformers/pull/4943?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM, thanks!