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/assistant/src/context.rs

@@ -549,7 +549,7 @@ impl Context {
         cx: &mut ModelContext<Self>,
     ) -> Self {
         let buffer = cx.new_model(|_cx| {
-            let mut buffer = Buffer::remote(
+            let buffer = Buffer::remote(
                 language::BufferId::new(1).unwrap(),
                 replica_id,
                 capability,