[BE] remove deprecated attributes from distributed_c10d (#105753)
Removing these attributes as they were introduced 5 years ago and before pytorch 1.0. `Backend` is the only support use now.
Differential Revision: [D47683717](https://our.internmc.facebook.com/intern/diff/D47683717)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105753
Approved by: https://github.com/rohan-varma
