chore: Fix some violations of 'needless_pass_by_ref_mut' lint (#18795)

While this lint is allow-by-default, it seems pretty useful to get rid
of mutable borrows when they're not needed.

Closes #ISSUE

Release Notes:

- N/A

crates/session/src/session.rs

@@ -64,7 +64,7 @@ pub struct AppSession {
 }
 
 impl AppSession {
-    pub fn new(session: Session, cx: &mut ModelContext<Self>) -> Self {
+    pub fn new(session: Session, cx: &ModelContext<Self>) -> Self {
         let _subscriptions = vec![cx.on_app_quit(Self::app_will_quit)];
 
         let _serialization_task = Some(cx.spawn(|_, cx| async move {