remove RobertaConfig inheritance from MarkupLMConfig
# What does this PR do?
Related to #19303

Removes `RobertaConfig` and `BertConfig` dependency from `MarkupLMConfig`. Even though `RobertaConfig` inherits from `BertConfig`, I have changed `MarkupLMConfig` to directly inherit from `PretrainedConfig`.

Added the following arguments in `__init__`:

- `bos_token_id = 0`
- `eos_token_id = 2`
- `position_embedding_type="absolute"`
- `use_cache=True`
- `classifier_dropout=None`

## 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 

_The documentation is not available anymore as the PR was closed or merged._<|||||>> Great work, thanks a lot! There is a typo in the docstring that is responsible for the failing test. I believe my suggestion should fix it :-)

Thank you for your suggestion.<|||||>Thanks for your work on this!