lint-doc check is a waste of energy

[talex5]

There's some discussion on https://discuss.ocaml.org/t/ocaml-org-recapping-2022-and-queries-on-the-fediverse/11099/22 about saving energy for the cluster.

I suggest removing the lint-doc stage. This is run for every single commit currently, and doesn't do anything (it runs with ODOC_WARN_ERROR=false which means that documentation errors don't fail the build).

Example: https://ci.ocamllabs.io:8100/job/2023-01-05/133221-ci-ocluster-build-c7323d