This reverts commit ef4beb8bc76f9d40b7e680cb0c2c7474d80f5a77.
It also removes the prior note about needing to patch `arcanist`.
This is patch is no longer necessary:
- arcanist came out of hibernation and updated to support PHP 8.1. https://github.com/phacility/arcanist/commits/master
- llvm/llvm-project is moving to github pull requests starting Sept 1 (tomorrow). https://discourse.llvm.org/t/pull-request-migration-schedule/71595
Fixes: #64470
Why do we use this fork?
https://github.com/phacility/arcanist has recently been updated?