Delete the 'Farming and food inspections' sector

<https://www.pivotaltracker.com/story/show/70255948>

The 'Farming and food inspections' specialist sector tags was created for the Rural Payments Agency, but it's no longer needed. 

This deletes the parent tag and its children. The 'down' method creates the tags again.

These tags never went live and nothing was ever tagged with it, so this is safe to delete without any further migrations elsewhere.