Remove deprecated StatementListHolder

[JeroenDeDauw]

[JeroenDeDauw]

I propose this is done in 8.0, as I suspect this could already have been done

[thiemowmde]

There is a tiny little bit of usage left in Wikibase.git, but we are getting rid of it right now: https://gerrit.wikimedia.org/r/343849. So: 👍