[SegFormer] Remove unused attributes
# What does this PR do?

This PR removes the unused attributes `image_size` and `downsampling_rates` from the configuration. As they simply aren't used by the model, this is not a breaking change.

It also adds a link to the blog post in SegFormer's docs, and clarifies that the model works on any input size as it pads the input.

It also improves the overall readability of `modeling_segformer.py`.
_The documentation is not available anymore as the PR was closed or merged._