[functorch] excise older custom_vjp prototype (#84638)
It was based off of the Python op registration API that has been
implemented in PyTorch already, so we can always bring it back, but
we're probably taking a different approach here.
Test Plan:
- tests
Differential Revision: [D39315050](https://our.internmc.facebook.com/intern/diff/D39315050)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84638
Approved by: https://github.com/samdow
