v1.8.20 — `vm_cancel_ttl` is not destructive — it prevents a deletion

The annotation was copied from vm_set_ttl directly above it, where
destructiveHint=True is right and carefully argued: set_ttl schedules an
unattended deletion that a daemon carries out later. cancel_ttl calls that off.

No behaviour changes; the CLI already asked nothing, correctly. What changes is
that a client reading the annotations is no longer told that preventing a
deletion is a destructive act.