'form_buffers' has been removed from ActiveAdmin::FormBuilder

I remove 'form_buffers' function because It has been removed from ActiveAdmin::FormBuilder in last version. 

See: 'https://github.com/activeadmin/activeadmin/blob/d775bb9edfc96c0d64bdde375fc3db697b77d728/lib/active_admin/views/components/active_admin_form.rb'