chore: remove legacy captive portal header

```rust
const NO_CONTENT_CHALLENGE_HEADER_LEGACY: &str = "X-Tailscale-Challenge";
```

needs to be removed in the next release

ref https://github.com/n0-computer/iroh/blob/18568ca9a2ce8cf227fea5d87081ea15ff976198/iroh-relay/src/server.rs#L59-L60