remove arg that is not being used
`extra_pos_embeddings` is passed but not being used anywhere, so deleting it.


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6931?src=pr&el=h1) Report
> Merging [#6931](https://codecov.io/gh/huggingface/transformers/pull/6931?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e95d262f2553859af9bffbfe5f5bc7e362259939?el=desc) will **increase** coverage by `0.42%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6931/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6931?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6931      +/-   ##
==========================================
+ Coverage   77.70%   78.12%   +0.42%     
==========================================
  Files         161      161              
  Lines       30119    30119              
==========================================
+ Hits        23403    23530     +127     
+ Misses       6716     6589     -127     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6931?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6931/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `94.00% <ø> (ø)` | |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6931/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.30% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6931/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6931/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `94.03% <0.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/6931/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `90.76% <0.00%> (+20.74%)` | :arrow_up: |

------

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<|||||>I think the optimal solution here is the deletion and then hardcode
`self.extra_pos_embeddings = 2` lower in the file.

Otherwise LGTM.<|||||>But it's there already in a different form: https://github.com/huggingface/transformers/blob/master/src/transformers/configuration_bart.py#L197
```
        self.extra_pos_embeddings = self.pad_token_id + 1
```
just to validate, you're suggesting to replace ` = self.pad_token_id + 1` with `= 2`, yes?<|||||>@stas00 yes! you can even delete the config attribute. and replace it with 2 in `modeling_bart.py` code. 

github wont let me suggest cause too low in file :)<|||||>I didnt know blenderbot was active. We may need this.
Don't merge yet pls.<|||||>I think I will need this for blenderbot, otherwise I'll reopen.