[Nix] Remove arcanist override of php8.0

Summary:
Since https://github.com/NixOS/nixpkgs/pull/182364 was merged,
this behavior is now the default in nixpkgs. We no longer need
to provide an override to change the behavior.

Test Plan:

# this can be used from any branch
nix develop github:commE2E/comm?ref=update-nix-flakes
# use any arc command, 8.1 would fail on `arc diff`

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4661