Remove duplicated extras["retrieval"]
The `extras["retrieval"]` is defined a few lines above as:
https://github.com/huggingface/transformers/blob/28b26013abea3a49afeb46d36993a568ec98f39e/setup.py#L217-L222
and then it seems to be overridden just below, probably linking to `faiss-cpu` being included even on windows.

This PR removes the second assignment. 

cc @LysandreJik @sgugger @stas00 