Remove unused CountdownEvent (#4290)

# Objective

Fixes:
#4287 

## Solution

I removed it.

Author: MiniaczQ

crates/bevy_tasks/src/countdown_event.rs

@@ -20,9 +20,6 @@ pub use single_threaded_task_pool::{Scope, TaskPool, TaskPoolBuilder};
 mod usages;
 pub use usages::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool};
 
-mod countdown_event;
-pub use countdown_event::CountdownEvent;
-
 mod iter;
 pub use iter::ParallelIterator;