Delete 08-maintenance.md

This information is being migrated to the Carpentries Handbook:

- [Maintainer Onboarding Process](http://docs.carpentries.org/topic_folders/maintainers/maintainers.html#maintainer-onboarding)
- [Lesson Release Process and Schedule](http://docs.carpentries.org/topic_folders/lesson_development/lesson_release.html)
- [GitHub Issue Tags](http://docs.carpentries.org/topic_folders/maintainers/github_labels.html)