Remove streamrpc implementation

As the Improve Git fetch data transfer efficiency project drifts to a new approach (gitlab-com/gl-infra/scalability#1216 (closed)) using yamux, the recent StreamRPC library code in the first iteration (introduced in !3601 (merged)) becomes obsoleted. We properly don't want to introduce and maintain a custom protocol anymore. This is heartbroken to remove, but we still need to do so, soon or later.

The library codes are isolated, not integrated into the production yet. Hence, I think it's safe to remove without any side effect.