RTF export at the moment exports children of group shapes as a flat
shape list. This means that the order inside the group shape is not
relevant for the toplevel ZOrder, so just don't write it.

Change-Id: I870707cb28c2f177c0e5d9cf5328260e76b661f3
Reviewed-on: https://gerrit.libreoffice.org/61805
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>