per-type table handled in content_storage

As dopry noted in his patch in http://drupal.org/node/107407#comment-934698, deleting rows in the per-type table could ba handled inide content_storage() rather than by additional code in content_delete() / content_delete_revision().

Attached patch takes care of this by always including the per type table in the [$field_name]['tables'] part of _content_type_info(), even if it holds no field.
This also lets us always add at least nid/vid in those tables.