Consider emoving warn-chaining

`warn-chaining` is too aggressive in detecting obsoleted use of task chaining without an explicit higher-order task because it only allows `do`. Any other higher-order task (like `thrush`) can legitimately use `group-args`, but currently everything but `do` will get a warning on improper chaining.

Since leiningen 2 has been around for a while now, I propose to simply remove the warning.