fix(codec): Remove Default bound on Codec

[andrewhickman]

Motivation

I implemented a custom codec to use with a dynamic method where the types are not known at compile time. However this bound is mildly annoying since there's no sensible way to implement default
https://github.com/andrewhickman/grpc-client/blob/487c5409e78afef200a064c81ae1ade1b37c3c28/src/grpc/codec.rs#L63

Solution

The bound is never used so we can just remove it. I think this is technically a breaking change for consumers of the Codec trait, however anything using ProstCodec, like generated code, still works.

[LucioFranco]

@davidpdrsn I forget did you remove the need for default when you changed some of the compression stuff?

[davidpdrsn]

Hm no I don't think so.

I tried bisecting it but had to go so far back that I hit

~/de/ma/tonic [HEAD @ 6dbe88d445e37 * bisect]
❯ cargo c --all-features --tests -p tonic
    Updating crates.io index
    Updating git repository `https://github.com/tower-rs/tower`
error: no matching package named `tower-balance` found
location searched: https://github.com/tower-rs/tower
required by package `tonic v0.1.0-alpha.6`

Seems fine to me to the Default requirement.