SSSS instead of seed cutting

## Context
In the chapter on [Redundant Metal Devices](https://github.com/BlockchainCommons/SmartCustodyWhitePapers/blob/master/%23SmartCustody-_Simple_Self-Custody_Cold_Storage_Scenario.md#-optional-step-use-metal-enhancement-redundant-metal-devices) you suggest to cut up the mnemonic in three parts, each containing two thirds of the mnemonic words.

## Problem with current approach
Don't roll your own crypto. Afaik, this is not best practice. The full 24 word seed has 256bit entropy, so each third of the seed has roughly 85bits.[?] If the attacker compromises two thirds of the seed from one backup, he only needs to brute force the last third with comparatively very little effort.

## Possible solution
How about a reference to Satoshilabs [SLIP-0039 : Shamir's Secret-Sharing for Mnemonic Codes](https://github.com/satoshilabs/slips/blob/master/slip-0039.md)?

Start with the Redundant Metal Devices section in #SmartCustody-_Simple_Self-Custody_Cold_Storage_Scenario.md and read the linked SLIP-0039 specification. Review the 13-comment discussion for the intended resolution. Done means the chapter no longer presents the disputed seed-cutting approach without addressing the concern, and accurately references the agreed alternative if appropriate.

Written by the indexing model from the issue text.