chore(nix): Remove treefmt

[srid]

We already use pre-commit and that supports nixpkgs-fmt.

cf. srid/haskell-template#144

Also, this PR updates git-hooks.nix while making a non-flake thereby reducing the number of lock file (dev/flake.lock) entries.

[shivaraj-bh]

The stdout when there is something to be formatted:

services-flake on  main [$!] via ❄️  impure (nix-shell-env) took 6s
❯ just fmt
pre-commit run -a
nixpkgs-fmt..............................................................Failed
- hook id: nixpkgs-fmt
- files were modified by this hook

flake.nix
1 / 8 have been reformatted
0 / 8 have been reformatted
0 / 8 have been reformatted
0 / 8 have been reformatted
0 / 8 have been reformatted
0 / 8 have been reformatted
0 / 8 have been reformatted
0 / 3 have been reformatted

error: Recipe `fmt` failed on line 29 with exit code 1

The error at the end is a bit misleading because it makes one think that formatting didn’t run successfully.

[srid]

I should have thought more on this.

pre-commit does not have a notion of "run all hooks unconditionally". Let's just get rid of the fmt target entirely, and briefly document pre-commit in the README like haskell-template now does:

srid/haskell-template@b224c24

Besides, fmt naming is now inaccurate anyway - since you can do more than formatting (e.g.: run hlint).