Revert "Remove unused callbacks in the Topic model"

This reverts commit b33ccaa.

That isn't hit by `git grep`, but actually used in meta-programming...

https://github.com/rails/rails/blob/b33ccaa6c335e2ce482c9de1aa05e4a612aa84bc/activerecord/test/cases/transactions_test.rb#L1020-L1028

Author: kamipo

activerecord/test/models/topic.rb

@@ -56,7 +56,9 @@ def topic_id
 
   alias_attribute :heading, :title
 
+  before_validation :before_validation_for_transaction
   before_save :before_save_for_transaction
+  before_destroy :before_destroy_for_transaction
 
   after_save :after_save_for_transaction
   after_create :after_create_for_transaction
@@ -106,7 +108,9 @@ def set_email_address
       end
     end
 
+    def before_validation_for_transaction; end
     def before_save_for_transaction; end
+    def before_destroy_for_transaction; end
     def after_save_for_transaction; end
     def after_create_for_transaction; end