The only reason why we deployed these files elsewhere in
tails_secrets_whisperback before, and then bind-mounted the destination to the
actual place where tor would look for these files, was to ensure we copied them
to an encrypted filesystem, back when we had systems without FDE, with a small
encrypted filesystem where we would store the secrets we thought about (while
other secrets would be stored in cleartext). I'm reasonably certain we will
not deploy tails::whisperback::relay on a system without FDE, without
giving it a second thought, so let's simplify.

Also, stop using tor::daemon::onion_service because its v3 support seems
broken to the point I think nobody ever tested it.

Finally, I initially wanted to distribute the Onion service secret and public
keys via Hiera, so we could fully deprecate the tails_secrets_whisperback
module, but their content is binary so I gave up on this.

This requires upgrading tor to stretch-backports on the monitoring master,
so that tails::monitoring::service::whisperback can connect to the v3 HS.