fdo#85542: fix DOCX import of overlapping bookmarks

This was broken by 345a3a394e082595924bf219796627f6c00ae2dd.
Kill the static variable and instead have some more state in the
implementation. Still not ideal, but at least fixes the regression.

Change-Id: I562f7d88a1983bd0ec2e01d6bb1e4a53551d0953
Reviewed-on: https://gerrit.libreoffice.org/12491
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>