Remove unused `activation_dropout` in OPT
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/18309

PR for corresponding models
* https://huggingface.co/facebook/opt-125m/discussions/19
* https://huggingface.co/facebook/opt-350m/discussions/5
* https://huggingface.co/facebook/opt-1.3b/discussions/7
* https://huggingface.co/facebook/opt-2.7b/discussions/5
* https://huggingface.co/facebook/opt-6.7b/discussions/8
* https://huggingface.co/facebook/opt-13b/discussions/7
* https://huggingface.co/facebook/opt-30b/discussions/8
* https://huggingface.co/facebook/opt-66b/discussions/7


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ArthurZucker 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ArthurZucker I am not sure who is the owner of the models repo but the following PRs also need to be merged.

> 
> https://huggingface.co/facebook/opt-125m/discussions/19
> https://huggingface.co/facebook/opt-350m/discussions/5
> https://huggingface.co/facebook/opt-1.3b/discussions/7
> https://huggingface.co/facebook/opt-2.7b/discussions/5
> https://huggingface.co/facebook/opt-6.7b/discussions/8
> https://huggingface.co/facebook/opt-13b/discussions/7
> https://huggingface.co/facebook/opt-30b/discussions/8
> https://huggingface.co/facebook/opt-66b/discussions/7
> <|||||>Hey, thanks for notifying, I will take care of it 😃 