Remove Bert's position_embeddings init in modeling_roberta.py
# What does this PR do?

Remove useless position_embeddings initialisation in modeling_roberta. 
First one was from Bert's code and second one is the tweaked Roberta's one.
Removed 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