`flake-parts`: Remove top-level 'treefmt' package

[srid]

See https://github.com/numtide/treefmt-nix/pull/14#issuecomment-1365944045

This effectively reverts #18

[marijanp]

I don't agree with the argument that it is intrusive to have the wrapped formatter available. I'm subscribing to the formatter-check anyways and therefore I want to have an easy way to access and run the formatter. now I have to duplicate the same code everywhere whether it be adding it to a devshell or as a flake output. in the first case requiring me to enter the devshell, if I don't use direnv, just to quickly run the formatter because the CI job failed.

[srid]

Some users dislike it, some users want it. We can make it an option so you only have to enable it rather than wire it all yourself, cf. autoWire = true; of https://github.com/srid/haskell-flake/issues/62

[zimbatm]

The new default is that it inserts itself as the default formatter invoked by nix fmt. And you can turn it off by setting flakeFormatter = false;. I think that's enough isn't it?

[Kranzes]

I much prefer it the way @zimbatm just explained.