Remove not used load hooks for active_record_config

These were removed with ActiveRecord::Model in
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#L15L156

activerecord/lib/active_record/core.rb

@@ -3,9 +3,6 @@
 require 'thread'
 
 module ActiveRecord
-  ActiveSupport.on_load(:active_record_config) do
-  end
-
   module Core
     extend ActiveSupport::Concern
 

activerecord/lib/active_record/timestamp.rb

 
 module ActiveRecord
-  ActiveSupport.on_load(:active_record_config) do
-  end
-
   # = Active Record Timestamp
   #
   # Active Record automatically timestamps create and update operations if the