searchable: refactor searchable_includes.

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

app/models/dtext_link.rb

@@ -30,7 +30,7 @@ class DtextLink < ApplicationRecord
   end
 
   def self.search(params)
-    q = search_attributes(params, :id, :created_at, :updated_at, :link_type, :link_target)
+    q = search_attributes(params, :id, :created_at, :updated_at, :link_type, :link_target, :model, :linked_wiki, :linked_tag)
     q.apply_default_order(params)
   end
 
@@ -48,10 +48,6 @@ class DtextLink < ApplicationRecord
     where(link_type: :wiki_link)
   end
 
-  def self.searchable_includes
-    [:model, :linked_wiki, :linked_tag]
-  end
-
   def self.available_includes
     [:model, :linked_wiki, :linked_tag]
   end