Delete penumbra_proto::serializers code

[hdevalence]

Is your feature request related to a problem? Please describe.

Long, long ago, Penumbra attempted to do "shaping" of JSON (e.g., encoding an address as a string). This was quickly discovered to be unworkable and a bad idea, because it meant that there was no longer a single source of truth (vs using ProtoJSON). However, the legacy of this code lives on in the serializers module, which should be deleted.

That code is still depended on as the source of truth for various Bech32m prefixes, which should move to the keys crate (or wherever else the relevant data is defined).