Fix for #3846
Fix for #3846 . PretrainedTokenizer mapped " do not" to " don't" when .decode(...) is called. Removed the " do not" --> " don't" mapping from clean_up_tokenization(...).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=h1) Report
> Merging [#4024](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ab90353f1abfd15f8d21f99395658d060679a08c&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4024/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4024      +/-   ##
==========================================
- Coverage   78.01%   77.93%   -0.09%     
==========================================
  Files         114      114              
  Lines       18671    18671              
==========================================
- Hits        14566    14551      -15     
- Misses       4105     4120      +15     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4024/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.36% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4024/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `90.47% <0.00%> (-2.47%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=footer). Last update [ab90353...333778c](https://codecov.io/gh/huggingface/transformers/pull/4024?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This looks good to me :-) Thanks for the PR!  What do you think @thomwolf @n1t0 ?<|||||>Hi @lukovnikov,

was the last commit by accident? I don't think that this one is needed to fix the issue no?<|||||>Thanks for pointing that out, should be good now.