when calling Link on SfxLinkItem, none of the callee sites free the
memory associated with the Clone()'ed SfxPoolItem, so just skip the
cloning, and make the Link take a const pointer to make this more
obvious.

Change-Id: Icd187e9d8d3ace9bf2e6ff183489999b105708e4
Reviewed-on: https://gerrit.libreoffice.org/55671Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>