Tidy up the code of events (#4713)

# Objective

- The code in `events.rs` was a bit messy. There was lots of duplication between `EventReader` and `ManualEventReader`, and the state management code is not needed.

## Solution

- Clean it up.

## Future work

Should we remove the type parameter from `ManualEventReader`? 
It doesn't have any meaning outside of its source `Events`. But there's no real reason why it needs to have a type parameter - it's just plain data. I didn't remove it yet to keep the type safety in some of the users of it (primarily related to `&mut World` usage)

Author: DJMcNab

benches/benches/bevy_ecs/run_criteria.rs

@@ -2,7 +2,7 @@ use bevy_ecs::{
     component::Component,
     prelude::{ParallelSystemDescriptorCoercion, Res, RunCriteriaDescriptorCoercion},
     schedule::{ShouldRun, Stage, SystemStage},
-    system::{IntoSystem, Query},
+    system::Query,
     world::World,
 };
 use criterion::{criterion_group, criterion_main, Criterion};