perf(page_service): dont .instrument(span.clone()) the response flush (#10686)

On my AX102 Hetzner box, removing this line removes about 20us from the
`latency_mean` result in

`test_pageserver_characterize_latencies_with_1_client_and_throughput_with_many_clients_one_tenant`.

If the same 20us can be removed in the nightly benchmark run, this will
be a ~10% improvement because there, mean latencies are about ~220us.

This span was added during batching refactors, we didn't have it before,
and I don't think it's terribly useful.

refs
- https://github.com/neondatabase/cloud/issues/21759

pageserver/src/page_service.rs

@@ -1280,8 +1280,6 @@ impl PageServerHandler {
                 }
                 Ok(())
             }
-            // and log the info! line inside the request span
-            .instrument(span.clone())
             .await?;
         }
         Ok(())