[examples] Cleanup summarization docs
Don't think we need a download_cnn_dailymail.py script

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=h1) Report
> Merging [#4876](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/02e5f79662d72cccdca81a47e3001a5f6d36e5b1&el=desc) will **decrease** coverage by `0.59%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4876/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4876      +/-   ##
==========================================
- Coverage   76.46%   75.86%   -0.60%     
==========================================
  Files         128      128              
  Lines       21502    21502              
==========================================
- Hits        16442    16313     -129     
- Misses       5060     5189     +129     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4876/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `28.15% <0.00%> (-63.03%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4876/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `94.73% <0.00%> (+10.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=footer). Last update [02e5f79...954557d](https://codecov.io/gh/huggingface/transformers/pull/4876?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
