Remove unused arguments in Multiple Choice example
In the dataset preparation, the arguments `pad_token_segment_id`, `pad_on_left`, `pad_token` and `mask_padding_with_zero` are inferred from the tokenizer to be sent to `convert_examples_to_features` which then does not use them (since `tokenizer.encode_plus` does all of this using the tokenizer state).

This PR cleans that up (and removes the TODO).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4853?src=pr&el=h1) Report
> Merging [#4853](https://codecov.io/gh/huggingface/transformers/pull/4853?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/37be3786cf1de9d21233f543c231866e68954998&el=desc) will **increase** coverage by `0.14%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4853/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4853?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4853      +/-   ##
==========================================
+ Coverage   76.40%   76.54%   +0.14%     
==========================================
  Files         128      128              
  Lines       21533    21533              
==========================================
+ Hits        16452    16483      +31     
+ Misses       5081     5050      -31     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4853?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4853/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.15% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4853/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.00% <0.00%> (+10.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4853?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/4853?src=pr&el=footer). Last update [37be378...9e1b14a](https://codecov.io/gh/huggingface/transformers/pull/4853?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Also, should the `DataProcessor` in this file simply use the one in `transformers`?<|||||>LGTM