Remove dead/useless code

Includes drop of cartodb:db:drop_trigger_cache_checkpoint rake task
Merge branch 'master-cartdofby-model-cleanups' of https://github.com/strk/cartodb into strk-master-cartdofby-model-cleanups

Conflicts:
	lib/tasks/db_maintenance.rake
Merge branch 'strk-master-cartdofby-model-cleanups'