Send NOFITY messages after deleting private-type records.

The `rndc signing -clear` command cleans up the private-type records
that keep track of zone signing activity, but before this change it
did not tell the secondary servers that the zone has changed.
73 jobs for !2909 with u/fanf2/signing-clear-notify in 34 minutes and 30 seconds (queued for 8 seconds)
latest merge request