test_tokenization_common.py: Remove redundant coverage
The deleted logic is already covered [here](https://github.com/huggingface/transformers/blob/3a58d6b22d436c71ee7511dbcae5128ed787ebf5/tests/test_tokenization_common.py#L1079)


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=h1) Report
> Merging [#6224](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0513f8d275022d4055b710a33cd520b2000982bf&el=desc) will **decrease** coverage by `1.30%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6224/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6224      +/-   ##
==========================================
- Coverage   79.61%   78.30%   -1.31%     
==========================================
  Files         146      146              
  Lines       26597    26597              
==========================================
- Hits        21175    20827     -348     
- Misses       5422     5770     +348     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.58% <0.00%> (-73.17%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.40% <0.00%> (-34.39%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+4.76%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `95.31% <0.00%> (+23.43%)` | :arrow_up: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (+35.21%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6224/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `88.19% <0.00%> (+63.97%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=footer). Last update [0513f8d...3a58d6b](https://codecov.io/gh/huggingface/transformers/pull/6224?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
