Remove `Roberta` Interdependency from `tokenization_luke`
# What does this PR do?

Related to #19303 
Removes `Roberta` dependency from `tokenization_luke.py`

## 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.
- [ ] 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?
@sgugger

> Thanks a lot for working on this one! Looking good for the properties added but the main init should retain the specificities of the Luke tokenizer, otherwise it will break it :-)

Hi,thanks for the super fast review. Will fix asap.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19349). All of your documentation changes will be reflected on that endpoint.<|||||>> I'm sorry if I was unclear in my previous comments. You still need to add the init of Roberta inside the init of Luke. It's just that you should not remove the extra code Luke adds in its init :-)
> 
> Also be careful with the docstring changes, they should be reverted.

working on it<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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