[BE] Rectify `C10_UNUSED_DISPATCH_CUDA_WORKAROUND` (#80423)
This was a workaround for CUDA-10.1 compiler bug
Which is not super relevant when CUDA-11.7 is out
Also, delete unused vars suppressions, as `C10_UNUSED` should be enough
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80423
Approved by: https://github.com/atalman
