brokerd: close the connection after the final frame again

hold_open (d21baa2) kept each connection open for up to two seconds
after its final frame, reading and dropping anything the peer sent, to
hide a test client that set a read timeout after the handler had closed.
On macOS that call fails with EINVAL; the clients now allow for it
(00a85c1, d7009dc), so the handler goes back to closing at once.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/brokerd/src/admin.rs

@@ -4,7 +4,7 @@
 use std::os::unix::net::UnixStream;
 
 use crate::approvals::Entry;
-use crate::broker::{Broker, forbid, hold_open, read_request, send};
+use crate::broker::{Broker, forbid, read_request, send};
 use crate::grants;
 use crate::ledger::{Answer, Answered};
 use proto::{
@@ -120,12 +120,10 @@ pub fn handle(mut stream: UnixStream, broker: &Broker) {
         }
         other => {
             forbid(broker, &mut stream, id, &other, "admin.sock");
-            hold_open(&mut stream);
             return;
         }
     };
 
     // 3. The final frame; a failed send is ignored.
     let _ = send(&mut stream, id, true, response);
-    hold_open(&mut stream);
 }