rust: pin-init: examples: fix incorrect drop

Remove the drop and associated clippy allow. The warning reported by Clippy
here is genuine; the binding created is `Pin<&mut T>` so dropping it does
nothing. `stack_pin_init` created bindings are only dropped at the end of
scope.

Reviewed-by: Benno Lossin <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Gary Guo <[email protected]>

rust/pin-init/examples/mutex.rs

@@ -91,7 +91,7 @@ impl<T> CMutex<T> {
     pub fn lock(&self) -> Pin<CMutexGuard<'_, T>> {
         let mut sguard = self.spin_lock.acquire();
         if self.locked.get() {
-            stack_pin_init!(let wait_entry = WaitEntry::insert_new(&self.wait_list));
+            stack_pin_init!(let _wait_entry = WaitEntry::insert_new(&self.wait_list));
             // println!("wait list length: {}", self.wait_list.size());
             while self.locked.get() {
                 drop(sguard);
@@ -99,9 +99,6 @@ impl<T> CMutex<T> {
                 thread::park();
                 sguard = self.spin_lock.acquire();
             }
-            // This does have an effect, as the ListHead inside wait_entry implements Drop!
-            #[expect(clippy::drop_non_drop)]
-            drop(wait_entry);
         }
         self.locked.set(true);
         unsafe {