look for collection-like fields that are never added to, and are
therefore effectively unused

Change-Id: Id52c5500ea5e3d2436fb5915aebb86278bf2d925
Reviewed-on: https://gerrit.libreoffice.org/60661
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>