Keep only Roberta's position_embeddings initialisation in modeling_roberta.py
# What does this PR do?

There was two position_embeddings initialisations, first one was left from Bert's code and second one is Roberta's tweaked version. Remove first one.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
@LysandreJik
_The documentation is not available anymore as the PR was closed or merged._