prevent scribd resubmit loop

fixes CNVS-11369

test plan
 - have a scribd doc that has been deleted
 - call Attachment.process_scribd_conversion_statuses
 - it should finish and not get stuck in a loop

Change-Id: I9a0e4199e1ee4fc41da7453d973b8db8e14450d0
Reviewed-on: https://gerrit.instructure.com/30450
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>