Add rake task to remove ServiceTier duplicates
Add rake task to remove ServiceTier duplicates

This allows us to remove duplicate entries for ServiceTiers. We accidentally created these duplicated entries through our ServiceInteraction import rake task which runs at the beginning of each month. There will a separate PR for updating the import code.

alphagov/local-links-manager,alphagov/local-links-manager,alphagov/local-links-manager