That one was an empty place-holder, BKE_collection_duplicate() is now
doing that. And its call from full scene copying is not needed
currently, 'deep copy' in that case is handled in editor's
`ED_scene_add()`.
Note that at some point, we might want to move that logic into BKE, but
for now let's keep thing as they are - working.
702e7bfd5c