campaigns: Remove unused changesets() query resolver
Created by: mrnugget
I just ran across this while looking at our resolver and noticed that there's no corresponding query in the GraphQL schema and that all the tests pass when I remove it.
I believe it's safe to remove it, can you confirm that @eseliger?
I'm not 100% sure whether my reasoning from "there is no explicit query with this name in the GraphQL schema" to "this is never called so we can remove it" is correct.