Remove add_column_with_default migration helper (#28654)

db/migrate/20181018205649_add_unread_to_account_conversations.rb

@@ -1,19 +1,15 @@
 # frozen_string_literal: true
 
-require Rails.root.join('lib', 'mastodon', 'migration_helpers')
-
 class AddUnreadToAccountConversations < ActiveRecord::Migration[5.2]
-  include Mastodon::MigrationHelpers
-
   disable_ddl_transaction!
 
   def up
     safety_assured do
-      add_column_with_default(
+      add_column(
         :account_conversations,
         :unread,
         :boolean,
-        allow_null: false,
+        null: false,
         default: false
       )
     end