Remove unnecessary use of FusedLayerNorm in XLNet
Fix #1172 for XLNet
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1434?src=pr&el=h1) Report
> Merging [#1434](https://codecov.io/gh/huggingface/transformers/pull/1434?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f3e0218fbb6bcc40b40f10089dae8876654edb23?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1434/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1434?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
- Coverage   84.72%   84.72%   -0.01%     
==========================================
  Files          84       84              
  Lines       12591    12590       -1     
==========================================
- Hits        10668    10667       -1     
  Misses       1923     1923
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1434?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/1434/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `72.09% <100%> (-0.05%)` | :arrow_down: |

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<|||||>Ok, thanks!