test: Remove python dead code linter

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.

Author: laanwj

ci/lint/04_install.sh

@@ -8,7 +8,6 @@ export LC_ALL=C
 
 travis_retry pip3 install codespell==1.15.0
 travis_retry pip3 install flake8==3.7.8
-travis_retry pip3 install vulture==1.0
 
 SHELLCHECK_VERSION=v0.6.0
 curl -s "https://storage.googleapis.com/shellcheck/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | tar --xz -xf - --directory /tmp/