Remove no longer needed call to default_scoped

Automatically using `default_scoped` in this context is the default
behavior in Rails 6.1.

config/initializers/acts_as_taggable_on.rb

@@ -5,8 +5,7 @@ module ActsAsTaggableOn
 
     scope :public_for_api, -> do
       where(
-        # TODO: remove default_scoped after upgrading to Rails 6.1
-        tag: Tag.default_scoped.where(kind: [nil, "category"]),
+        tag: Tag.where(kind: [nil, "category"]),
         taggable: [Debate.public_for_api, Proposal.public_for_api]
       )
     end