Clean up code base, remove unused code and tests

[tmfrnz]

Drop SDG target tables (not needed in this multiframework version):

• [ ] tables: sdgtarget_categories, sdgtarget_indicators, sdgtarget_measures, sdgtarget_recommendations, sdgtargets
• [ ] controllers
• [ ] models
• [ ] policies
• [ ] serialisers
• [ ] specs: controllers, factories, models
• [ ] config/routes

Remove any unused Views:

• [ ] users
• [ ] devise?
• [ ] any other?

Remove unused specs

• [ ] if any?

[tmfrnz]

• [ ] remove unused gems (batch_api ? )
• [ ] update gems (if advisable)