Mark CVE-2017-17689 as unfixed for unstable

The original bug #898631 was for both S/MIME and PGP/MIME part actually,
but the S/MIME part is not yet fixed for thunderbird. Nor is the CVE
considered as fixed by mozilla itself. Revert thus the fixed status. For
now not reopening the bug, we might track a new one in the BTS
specifically for the S/MIME part.

We might ignore the issue in future given there is conceptual flaw in
S/MIME.