fix: print enum variant name for RPC debug logs (#1503)

## Description

Right now in the debug logs we print the following for each incoming RPC
request:
```
handling rpc request: quic_rpc::transport::flume::FlumeServerEndpoint<iroh::rpc_protocol::ProviderRequest, iroh::rpc_protocol::ProviderResponse>
```
which is very long and not useful because it contains no info at all
about incoming request.

Before that, we printed the full Debug impl, which contained too much
(especially bytes).

This PR changes it to just print the enum variant name, so e.g.

```
handling rpc request: BlobRead
```

Also removes a debug leftover `println`.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.

Author: Frando