Removing one of the twice defined position_embeddings in LongFormer
# What does this PR do?

The self.position_embeddings in LongFormerEmbeddings is defined twice. Removing the first without padding_idx
l. 451/452

- [X] 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]

## Who can review?
- text models: @ArthurZucker and @younesbelkada

_The documentation is not available anymore as the PR was closed or merged._