Detox, part 2 of N (#12152)

I've argued in #11537 that poetry and tox don't cooperate well at the
moment. (See also #12119.) Therefore I'm pruning away bits of tox to make the transition to poetry easier. This change removes the commands for coverage.

We don't use coverage in anger at the moment. It shouldn't be too hard to add coverage as a dev-dependency and reintroduce this if we really want it.