Primarily I'd like to remove this because it is very imprecise, due to
Python's dynamic nature, giving it a large list of false positives that
need to be listed as exceptions. See for example #16906.
It's also a frequent source of complaints. I'm doubtful of the
usefulness of checking for dead code in a linter in the first place.
Having some dead code in the test framework for a while is not a
disaster.
