related fdo#65430: forgot to 'unlink' setOpenTemplateHdl

This was introduced in 96e7225260aac0815643a4043f4ddb4e6b5764a1

Change-Id: I9bb8abbc9e9167b1e43bebf4477cf73010bf2d92
Reviewed-on: https://gerrit.libreoffice.org/7958
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>