Fix segformer reshape last stage
# What does this PR do?

This PR improves SegFormer by removing the `reshape_last_stage` attribute of the configuration. In fact, this attribute was not needed at all. 

Previously, the `reshape_last_stage` argument was set to `False` for `SegformerForImageClassification` models, however it's perfectly fine to reshape afterwards within this head model. 

@sgugger I know you'll warn me about this being a breaking change. Here's why I think this change will not have an impact:

- `reshape_last_stage` was set to `True` by default in the configuration. As this PR only removes the ability to set this to `False`, the only use case in which this might be a breaking change is in case one defined a `SegformerModel` with `config.reshape_last_stage = False`. I am not sure if anyone has done this because there are no use cases for this.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15748). All of your documentation changes will be reflected on that endpoint.