Don't delete trust on backup stack delete

Since the trust_id is copied from the stack being backed-up, we
don't want to delete the trust when the backup is deleted, or
the actual stack will break when the next deferred auth operation
occurs.

Closes-Bug: #1271968
Change-Id: I2b971fad7e010469cbb8a9c9ed326d89e92f2197
Merge "Don't delete trust on backup stack delete"