Remove misplaced summarization documentation
Documentation for the previous version of abstractive summarization is still present in the repository:

https://twitter.com/DavidMezzetti/status/1204123548966621184

This PR removes it.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2122?src=pr&el=h1) Report
> Merging [#2122](https://codecov.io/gh/huggingface/transformers/pull/2122?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e57d00ee108595375504eb21c230ce35428aae5e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2122/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2122?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2122   +/-   ##
=======================================
  Coverage   80.08%   80.08%           
=======================================
  Files         112      112           
  Lines       16862    16862           
=======================================
  Hits        13504    13504           
  Misses       3358     3358
```



------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2122?src=pr&el=footer). Last update [e57d00e...a5fa0de](https://codecov.io/gh/huggingface/transformers/pull/2122?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
