don't export draft children of orphaned drafted (TNL-923)

@dmitchell 
@zubair-arbi 

Fixes an issue where the draft children of orphaned drafts were being exported

related bugs:
https://openedx.atlassian.net/browse/TNL-923
https://openedx.atlassian.net/browse/TNL-872
No, that shouldn't be an issue, because instead of deleting children of orphans, we're just ignoring them. If we have a case where a child html draft is pointed to by an orphan vertical draft and a normal vertical draft, the child will get exported as part of exporting the normal vertical draft.

Before this fix, we would have exported that child html draft twice actually, once with the normal vertical draft, and once with the orphan vertical draft.