searchable: refactor searchable_includes.

Pass searchable associations directly to search_attributes instead of
defining them separately in searchable_includes.

app/models/moderation_report.rb

@@ -82,16 +82,12 @@ class ModerationReport < ApplicationRecord
   end
 
   def self.search(params)
-    q = search_attributes(params, :id, :created_at, :updated_at, :reason)
+    q = search_attributes(params, :id, :created_at, :updated_at, :reason, :creator, :model)
     q = q.text_attribute_matches(:reason, params[:reason_matches])
 
     q.apply_default_order(params)
   end
 
-  def self.searchable_includes
-    [:creator, :model]
-  end
-
   def self.available_includes
     [:creator, :model]
   end