[REVIEW] Removed duplicate XLMConfig, XLMForQuestionAnswering and XLMTokenizer in run_squad.py
Before this PR, in run_squad.py at lines 58-65 were the following:
```
MODEL_CLASSES = {
    'bert': (BertConfig, BertForQuestionAnswering, BertTokenizer),
    'xlnet': (XLNetConfig, XLNetForQuestionAnswering, XLNetTokenizer),
    'xlm': (XLMConfig, XLMForQuestionAnswering, XLMTokenizer),
    'distilbert': (DistilBertConfig, DistilBertForQuestionAnswering, DistilBertTokenizer),
    'albert': (AlbertConfig, AlbertForQuestionAnswering, AlbertTokenizer),
    'xlm': (XLMConfig, XLMForQuestionAnswering, XLMTokenizer)
}
```

After this PR, I've **removed** the last (key, value) pair in the `MODEL_CLASSES` dictionary because it contains _xlm_ as key **two** times.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2224?src=pr&el=h1) Report
> Merging [#2224](https://codecov.io/gh/huggingface/transformers/pull/2224?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8efc6dd544bf1a30d99d4b5abfc5e214699eab2b?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/2224/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2224?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2224   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files         122      122           
  Lines       18344    18344           
=======================================
  Hits        14946    14946           
  Misses       3398     3398
```



------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2224?src=pr&el=footer). Last update [8efc6dd...e6a7670](https://codecov.io/gh/huggingface/transformers/pull/2224?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks @TheEdoardo93 !