Tx Comments - Either remove or hide

The section https://docs.celo.org/home/protocol/transactions/tx-comment-encryption

It describe how `Accounts` contract hold an encrypting key, but it does not describe how to use this.

Need to review, but i believe this was only possible on Mento's StableToken implementation, and unclear if it still possible.

So, feels like a 'abandoned' feature, and we can keep it on legacy docs, but most probable remove this.

Need to chat with primitives team on this

cc @pahor167