searchable: refactor searchable_includes.

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

app/models/post_approval.rb

@@ -38,14 +38,10 @@ class PostApproval < ApplicationRecord
   end
 
   def self.search(params)
-    q = search_attributes(params, :id, :created_at, :updated_at, :post, :user)
+    q = search_attributes(params, :id, :created_at, :updated_at, :user, :post)
     q.apply_default_order(params)
   end
 
-  def self.searchable_includes
-    [:user, :post]
-  end
-
   def self.available_includes
     [:user, :post]
   end