Remove c10->ATen registration forwarding (#28186)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28186
Since now all ops are on c10, we don't need to forward any registrations to globalATenDispatch anymore.
ghstack-source-id: 92586962
Test Plan: waitforsandcastle
Differential Revision: D17969011
fbshipit-source-id: 30e6cb072c934b3d24089055754ed3695f8ea693
