Clean up PreserveOriginal comments
Clean up PreserveOriginal comments

One of the is fine, but better placed closer to the reference.

The other doesn't relate to that code path at all, and is already
documented in types.BlobInfo.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image