purge 'stats' subcommand

Is anything consuming the text files in fdroiddata/stats/? Now that fdroiddata is all YAML, it provides an API to provide things like disabled_apps, repotypes, update_check_modes, categories, and licenses. Then index-v1.json provides antifeatures, knownapks, and latestapps.

• !803 (closed) leads towards replacing knownapks entirely
• !794 (merged) latestapps is no longer used in fdroidserver