Remove Attachment#to_param
Remove Attachment#to_param

This avoids the following error when creating/updating attachments:

> undefined method `empty?' for
> BSON::ObjectId('591ad57fedf8775831000004'):BSON::ObjectId
> (ActionView::Template::Error)

The error was raised when passing an instance of Attachment to
`link_to`.

The `#to_param` method was added in
3b2d5c8ae14c1132a6bf04d702bd663db29408ff but no longer appears to be
necessary.

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher