Use DecryptVerifyJob to decrypt and/or verify inline PGP message blocks

The old code was unnecessarily complicated. And it failed to work with
GpgME 1.24 because the verify jobs now explicitly ask gpg to perform
a signature verification and that causes a failure if the data isn't
signed.

(cherry picked from commit 1603fb1fe54dc8b55646d358eccbaf2f020aa9c1)