998631-task-statuses - clear task statuses references before deleting

Prevents the following error

/opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:1158:in `async_exec': PGError: ERROR:  update or delete on table "task_statuses" violates foreign key constraint "providers_task_status_id_fk" on table "providers" (ActiveRecord::InvalidForeignKey)
DETAIL:  Key (id)=(1) is still referenced from table "providers".
: DELETE FROM "task_statuses"