Clean up LWECiphertext Attributes

In 2/9 office hours, we discussed a pain point related to KeySwitchKey types: since they have additional "non-level" moduli, the ModulusChain attribute doesn't really work well for them. An LWEPublicKey would be more accurate, but we're missing lots of ops on PublicKeys that we would have to add.

A better approach would be to remove unnecessary attributes from LWECiphertexts. We think that PlaintextSpace, ModulusChain, and ApplicationData attributes are probably no longer needed. For this ticket: validate this hunch, remove unneeded LWECiphertext attributes, and either unify LWECiphertext with LWEPublicKey or make them interoperable.