Delete sdgtarget features

[lukearndt]

Context

#363

We don't need the SDG Target features so we want to delete them

Changes

This commit deletes the SDG Target features from the application. This
includes the following tables and all of their corresponding
associations, controllers, factories, models, policies, routes,
serializers, and specs:

• sdgtarget_categories
• sdgtarget_indicators
• sdgtarget_measures
• sdgtarget_recommendations
• sdgtargets

It does not delete the groups_sdgtargets_default field from the
Taxonomy model.

Considerations

The database migration that removes the tables is reversible, with the
following caveats:

1. It recreates the dropped tables using the statements in schema.rb,
   which were taken from the version in nz-justice-development at the
   time of this commit. Right now, there aren't any pending PRs that alter
   those table definitions so they should be correct, but that could change
   between the creation and merging of this PR.

2. Rolling back the migration will restore the tables, but it will not
   restore any of their previous data. That data will only be recoverable
   if we take and retain a database backup prior to deploying this change.