This fixes build [1]

This fails thanks to D3635 which revealed that the type is no longer correct in
that code!

This code is:

• not covered
• no longer used

I'm just making it type so the build gets back to its feet.

It got used once to update our contents with the new blake2s256 column.

Wondering whether or drop it (we could drop the storage.content_update
as well as it is the only client). I gather if we need something similar in the future,
we could use a tested journal implementation instead (it worked ok at the times heh)

[1] https://jenkins.softwareheritage.org/job/DCIDX/job/tests/1075/console

tox