[Longformer] Remove redundant code
This PR fixes class  LongformerSelfAttention as follow:

1. Since the method  **_mask_invalid_locations()** was already run in  **_sliding_chunks_matmul_qk()**, it should be removed from **forward()** to avoid code duplication.

2. In the method  **_mask_invalid_locations()**,  since the size of the variable **beginning_mask** is (1, w, 1, w+1) and w is always less than **seqlen**, the index **beginning_mask[:, :seqlen]** is needless.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4839?src=pr&el=h1) Report
> Merging [#4839](https://codecov.io/gh/huggingface/transformers/pull/4839?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e33fdc93b4ecb571dd7a8002a74789ec8bfffc09&el=desc) will **decrease** coverage by `0.23%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4839/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4839?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4839      +/-   ##
==========================================
- Coverage   76.17%   75.94%   -0.24%     
==========================================
  Files         128      128              
  Lines       21497    21495       -2     
==========================================
- Hits        16375    16324      -51     
- Misses       5122     5171      +49     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4839?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `92.99% <100.00%> (-0.04%)` | :arrow_down: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `71.83% <0.00%> (-13.93%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.00% <0.00%> (-1.36%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `89.91% <0.00%> (-0.25%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4839/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.36% <0.00%> (+0.31%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4839?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/4839?src=pr&el=footer). Last update [e33fdc9...6eb5f7d](https://codecov.io/gh/huggingface/transformers/pull/4839?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM! All the `RUN_SLOW=1` tests pass. Also pinging @ibeltagy to make sure.<|||||>LGTM<|||||>looks good to me. Thanks, @ZhuBaohe.

> w is always less than seqlen

Just wanted to mention that this is true only because of the [padding](https://github.com/huggingface/transformers/blob/6eb5f7d3441c2eb768da8f70dc8a602c02468267/src/transformers/modeling_longformer.py#L647). 
