Add 'Vulture' to our automated testing

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1072)
In GitLab by [[Gitlab user @aevri]](https://gitlab.com/aevri) on Jul 9, 2019, 13:16

To avoid introducing unused code, and other accidents, use [vulture](https://pypi.org/project/vulture/) to detect it for us.

We can add this to our automated tests, perhaps after removing more dead code and ignoring some false positives.

This is a continuation of the effort in !1455.

Review the automated test configuration and the prior !1455 effort, then identify where Vulture should run and which false positives need handling. Done means the automated tests run Vulture reliably without introducing unresolved findings.

Written by the indexing model from the issue text.