now that we have upgraded to VS2017, we can use std::unique_ptr in
std::map

Change-Id: Id01af07ccae7447405b8f0bc44b08043f453e54b
Reviewed-on: https://gerrit.libreoffice.org/57384
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>