(#16376) Fix rails compatibility layer for activerecord < 3.0

Previously an alias was attempted between scope and named_scope to allow older
activerecord versions (< 3.0) to work with the inventory service while also
supporting modern activerecord. The scope alias failed because
Puppet::Rails::InventoryNode inherits from ActiveRecord::Base, which has a
scope method already. So activerecord's scope method is found instead of the
intended alias target. This commit removes the alias and simply switches on
activerecord version to use the correct scope (AR >= 3.0) or named_scope (AR <
3.0) method.