fix: remove shutdown_flag param to run_login_server() (#2399)

In practice, this was always passed in as `None`, so eliminated the
param and updated all the call sites.

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[//]: # (BEGIN SAPLING FOOTER)
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* __->__ #2399
* #2398
* #2396
* #2395
* #2394
* #2393
* #2389

codex-rs/login/src/server.rs

@@ -77,10 +77,7 @@ impl ShutdownHandle {
     }
 }
 
-pub fn run_login_server(
-    opts: ServerOptions,
-    shutdown_flag: Option<Arc<tokio::sync::Notify>>,
-) -> io::Result<LoginServer> {
+pub fn run_login_server(opts: ServerOptions) -> io::Result<LoginServer> {
     let pkce = generate_pkce();
     let state = opts.force_state.clone().unwrap_or_else(generate_state);
 
@@ -118,7 +115,7 @@ pub fn run_login_server(
         })
     };
 
-    let shutdown_notify = shutdown_flag.unwrap_or_else(|| Arc::new(tokio::sync::Notify::new()));
+    let shutdown_notify = Arc::new(tokio::sync::Notify::new());
     let server_handle = {
         let shutdown_notify = shutdown_notify.clone();
         let server = server.clone();