only delete scribd_docs from self

don't delete scribd_docs when attachments are destroyed

fixes CNVS-11923

test plan
 - have attachment with scribd_doc  that has
   root_attachment with scribd_doc
 - call delete_scribd_doc on child
 - parent scribd_doc should be un-touched

Change-Id: I1cbac580e40c6b8f5c2a8612f5390dfae681d9c4
Reviewed-on: https://gerrit.instructure.com/32123
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>