Create one-off scheduled task to delete old OTKs (#17934)

To work around the fact that,
pre-https://github.com/element-hq/synapse/pull/17903, our database may
have old one-time-keys that the clients have long thrown away the
private keys for, we want to delete OTKs that look like they came from
libolm.

To spread the load a bit, without holding up other background database
updates, we use a scheduled task to do the work.