Guard Wheel.remove against expired instant removals

Trying to remove an already expired Timer Wheel entry (called by
DelayQueue.reset()) in some cases as described in (tokio-rs#2573) causes panics.

This prevents this panic by omitting removal in these cases

tokio/src/time/wheel/mod.rs

@@ -126,9 +126,11 @@ where
     /// Remove `item` from thee timing wheel.
     pub(crate) fn remove(&mut self, item: &T::Borrowed, store: &mut T::Store) {
         let when = T::when(item, store);
-        let level = self.level_for(when);
 
-        self.levels[level].remove_entry(when, item, store);
+        if when > self.elapsed {
+            let level = self.level_for(when);
+            self.levels[level].remove_entry(when, item, store);
+        }
     }
 
     /// Instant at which to poll