MB 23 is no longer needed as an updated proposal MB 25 was executed via the whitelist track.

Cancel an ongoing referendum.

- `origin`: must be the `CancelOrigin`.
- `index`: The index of the referendum to be cancelled.

Emits `Cancelled`.

- Proposal MB 23 is not needed anymore because a new proposal called MB 25 was already approved.
- MB 25 was executed through the whitelist track.
- The details of MB 23 are no longer relevant.
- The new proposal is called MB 25.
- The new proposal was approved through the whitelist track.

#29 Cancel MB23