Remove the ability to serialize a plaintext

[crockeea]

We should remove repeated double raw_pt = 4; // raw (unscaled) plaintext encrypted in the ciphertext; only non-empty from the ciphertext protobuf object so that it's harder for users to do something insecure.

Any debugging that the user wants to do can be done locally (without serialization), so this doesn't seem to be necessary from a functionality perspective.