[functorch] Get rid of defunct functorch/setup.py (#87235)
We initially left it there for BC concerns.
- It has been more than a month since then,
- I have migrated folks who used the previous install command (pip
install ...pytorch.git@subdir=functorch) off of it
so it's time to get rid of it
Test Plan:
- code reading
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87235
Approved by: https://github.com/Chillee
