Delete the 'Farming and food inspections' sector

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.

It never went live and nothing was ever tagged with it, so this is safe 
to delete without any further migrations elsewhere.
Merge pull request #159 from alphagov/delete-rpa-sector

Delete the 'Farming and food inspections' sector

https://www.pivotaltracker.com/story/show/70255948