Its only client was hive-c0re's matrix-account-login handler, removed
earlier on this branch, so nothing sends `restart_matrix_daemon` any
more. Drops the `PrivRequest` variant, the hive-priv handler and its
`systemctl --machine=h-<agent> restart hive-matrix-daemon.service`
helper, and the row in the hive-priv op table in security.md.
`PrivRequest` is internally tagged by `op` name, so no other variant's
encoding changes. A new token file still re-fires the daemon through
its `matrix-token*` path unit.
Refs #4348
1d8ec00ddc