Urgh. I hadn't realized that `ProtobufDecoder` would always get created, even if another name was explicitly used, but there it is in the code, plain as day.

The original intent was to have the most common useful setup be there by default, but to allow the users to override that w/ custom decoder setups if they wanted to. Sounds like we succeeded w/ the first goal, but failed somewhat w/ the second.

Now that the decoder usage patterns have changed a bit (i.e. there's no more "expected set of decoders" implied by a Heka protocol enum), I'm inclined to agree that we should stop creating the ProtobufDecoder by default and require that it be explicitly added and used, like the rest.