Minor dead code clean-up
Hello,

I am not sure how sensitive you generally are about dead code in the repository. I have identified a few places with dead code, where I believe a clean-up would improve readability.

- removal of a couple of unused dropouts I came across in Albert and XLNet
- removal of an unused code block for relative attention shift for XLNet

## Who can review?

@LysandreJik , @TevenLeScao 

Closing as it seems the changes were added in another PR