Remove register_mobile_autograd.cpp. (#42397)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42397
Since the autograd registration is unified to code-gen, we don't need to keep a manual registration file for mobile.
Remove it to avoid extra maintenance.
Test Plan: Imported from OSS
Reviewed By: ljk53
Differential Revision: D22883153
Pulled By: iseeyuan
fbshipit-source-id: 6db0bd89369beab9eed6e9a9692dd46f5bd1ff48
