now that UpdateBuild RPC is available, use it to finalize a LUCI build from kitchen. This means kitchen has full control over the final result and buildbucket no longer needs to interpret its kitchen-specific output. It also means swarming task result is also no longer important. We no longer have to merge kitchen output with task result. Strictly speaking, we no longer have to wait for swarming task completion, including running the OOming cronjob that collects all task results.