Remove the AI severity from one artifact, so it reports the severity its sandflies give it again.

The host's AI rating is withdrawn with it. An operator who restores one artifact has overruled part of the verdict's rating, so until a new verdict is formed the host's severity is the highest severity among its alerting artifacts, and the verdict reports severity_withdrawn. Neither the host's verdict nor the artifact's own record is deleted, and a later verdict for the host may set the severity again.

Idempotent: reverting an artifact that has no AI severity succeeds, changes nothing -- the host's rating included -- and reports that there was nothing to revert, so a bulk action does not have to know which rows have one.