distilberttokenizer.encode_plus() token_type_ids are non-default
DistilBert doesn't use token_type_ids. Therefore the encode_plus() method of the DistilBertTokenizer should generate them per default. This fix sets the default value of return_token_type_ids to False.

Closes #2702
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2870?src=pr&el=h1) Report
> Merging [#2870](https://codecov.io/gh/huggingface/transformers/pull/2870?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/73028c5df0c28ca179fbe565482a9c2143787f61?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2870/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2870?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2870      +/-   ##
==========================================
+ Coverage   75.06%   75.06%   +<.01%     
==========================================
  Files          94       94              
  Lines       15288    15290       +2     
==========================================
+ Hits        11476    11478       +2     
  Misses       3812     3812
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2870?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2870/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `86.1% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2870/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZGlzdGlsYmVydC5weQ==) | `100% <100%> (ø)` | :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/2870?src=pr&el=footer). Last update [73028c5...db509eb](https://codecov.io/gh/huggingface/transformers/pull/2870?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>obsolet since 2.6 release