Remove mutation score strategy from the build (pitest/descartes)

I'm really sad to be working on this but we've come to the conclusion that it's currently doing us more harm than good. See https://forum.xwiki.org/t/should-we-keep-pitest-descartes-in-our-build/6864/8

We could put it back in the future when it becomes more stable and there are no more flickers. This issue will serve to locate all the commits used to remove the feature so that it's easy to undo it in the future if need be.