executor: drop the MaRS service

This service is already an implementation detail of the executor, and the usefulness of having a separate service just to store the equivalent of a couple of kB of configuration is questionable.

Additionally, using sqlite to store what could essentially be a configuration file makes backups harder to diff.

This commit drops MaRS, and replaces the database with a simple YAML file. This file gets reloaded before every edit, but it will need to get auto-reloaded in a future commit so that user-edits get reflected immediately.