ASYM_ENCRYPT: deprecate sign_msgs

In ASYM_ENCRYPT, signing messages means that the checksum of an encrypted message is computed and used together with the secret key of the sender to sign the message. On the receiver side, the public key of the sender is used to validate the signature.

However, this is redundant, as decryption of a message will fail if the contents have been changed.

If needed, signing of messages can be done in a separate protocol.