Removes Roberta and Bert config dependencies from Longformer
# What does this PR do?

@sgugger , 

Per the issue #19303, the Roberta and Bert config dependencies are removed from `LongformerConfig` and it now directly inherits from `PretrainedConfig`.  

- `LongformerConfig` depends on `RobertaConfig` and `RobertaConfig` in turn inherits from `BertConfig`. 
-  So I've copied over the defaults (`pad_token_id`, `bos_token_id`, `eos_token_id`) from RobertaConfig and the rest of the defaults from BertConfig that are not conflicting with roberta.  
- The docstrings from BertConfig are copied over as well. 


## 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).
- [x] Did you write any new necessary tests?

_The documentation is not available anymore as the PR was closed or merged._<|||||>Done! Thanks for bearing with me @sgugger :) 