[testing] remove USE_CUDA
As discussed at https://github.com/huggingface/transformers/issues/6349 this PR removes `USE_CUDA`.

I don't think I needed to add `CUDA_VISIBLE_DEVICES=""` to `.circleci` config files since those CIs have no gpus anyway.

@LysandreJik, @sgugger, @sshleifer 

fixes: #6349