remove an unused type from the reentrant lock tests

Author: RalfJung

library/std/src/sync/reentrant_lock/tests.rs

@@ -51,10 +51,3 @@ fn trylock_works() {
     .unwrap();
     let _lock3 = l.try_lock();
 }
-
-pub struct Answer<'a>(pub ReentrantLockGuard<'a, RefCell<u32>>);
-impl Drop for Answer<'_> {
-    fn drop(&mut self) {
-        *self.0.borrow_mut() = 42;
-    }
-}