fix(nvim): remove unused mut variables

nvim-core/src/headless.rs

@@ -27,7 +27,7 @@ pub async fn spawn_headless_nvim() -> Result<String, String> {
 
     tokio::time::sleep(tokio::time::Duration::from_millis(1000)).await;
 
-    let mut old_child = {
+    let old_child = {
         let mut lock = HEADLESS_PROC.lock().unwrap_or_else(|e| e.into_inner());
         let old = lock.take();
         *lock = Some(child);