Remove *-index-only challenges

[danielmitterdorfer]

Our tracks contain challenges that index a data set and then perform additional tasks such as queries. These are usually called append-no-conflicts. Additionally, there are challenges that only index a data set, usually called append-no-conflicts-index-only. As we have flexible task filters, there is no need for this duplication and we should therefore deprecate all *-index-only challenges with a message indicating the preferred behavior (mentioning that users should use append-no-conflicts and task filters) and then remove them after a short grace period (2 weeks?).