don't copy scribd attributes from the root_attachment

test plan:
 - have a course with working scribd documents
 - copy the course
 - documents should still be viewable in the copy
 - if the document is deleted from scribd due to non-use,
   viewing it in either course should trigger a
   re-render

fixes CNVS-10818

Change-Id: Ideb19e5e84575a63727bd2ec78790fa7c1d87f16
Reviewed-on: https://gerrit.instructure.com/30625
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>