Fix issue with not being able to delete doc series

before_destroy does nothing as it's covered by a guard

https://www.pivotaltracker.com/s/projects/367813/stories/48855141
Merge pull request #601 from alphagov/48855141-fix-doc-series-delete

Fix issue with not being able to delete doc series

https://www.pivotaltracker.com/s/projects/367813/stories/48855141