Remove a chunk of O(n^2) work in XMLHints_Impl

Change-Id: I6b391af630d83ddd563b66bc1ad1640cd78debc7
Reviewed-on: https://gerrit.libreoffice.org/72948
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>