Re-evaluate the `SecureLink.encryptionMethod` property

Created by: absoludity

I'm splitting this off from a comment in https://github.com/uncefact/spec-untp/issues/408 :

encryptionMethod - though it's not clear to me how we were planning to use encryptionMethod with a link here

Having looked through the documentation, it appears to only have one option when specified: AES . AFAICT, if specified as "AES", it indicates that the target of the link is encrypted via AES.

First, this seems redundant - it will be clear that the data is encrypted. Given that the spec says the key is provided out-of-band, the type of encryption can be provided with the key rather than here. So I don't think this property is required at all.

Second, I think there's a larger issue with symmetric key encryption being used to protect distributed data - I've created #469 for that since even if we remove encryptionMethod here, that larger issue of how to provide access control is still relevant and needs to be discussed.

For this issue alone here, I don't think encryptionMethod is serving any purpose and it is stopping us from moving to a digestMultibase field for secure links (see #408).