fields are internal bookmarks. When the exporter runs
through the bookmarks, it will already write out a bookmark
entry, so don't output a separate one for the fieldmark.

Change-Id: I84af2989035507ac745d028f1585d60d8823ff8b
Reviewed-on: https://gerrit.libreoffice.org/61616
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>
(cherry picked from commit 0a452b50)