Takes load time from 22s to 15.5s

sw::mark::IMark does not need to be held by shared_ptr - we have one
vector, which holds all marks we created, and when we remove from that
vector, which remove the other references too.

Change-Id: Ie6d287d9d825f7129855b64a34afa81a2ef9c378
Reviewed-on: https://gerrit.libreoffice.org/73206
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>