It removes diagram content and rebuilds shape tree from diagram markup.
Diagram should appear unchanged - editing will be added in future commits.

Change-Id: I6bfc21956a1a23e55d3c7a74384db71d957d690d
Reviewed-on: https://gerrit.libreoffice.org/71584
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>