Clean up WinitWindows::remove_window (#12749)

# Objective

- Avoid unbounded HashMap growth for opening/closing windows.

## Solution

- Remove map entry in `WinitWindows::remove_window`.

## Migration Guide

- `WinitWindows::get_window_entity` now returns `None` after a window is
closed, instead of a dead entity.

---

## Comments

The comment this PR replaces was added in
https://github.com/bevyengine/bevy/pull/3575. Since `get_window_entity`
now returns an `Entity` instead of a `WindowId`, this no longer seems
useful. Note that `get_window_entity` is only used
[here](56bcbb0975/crates/bevy_winit/src/lib.rs (L436)),
immediately followed by a warning if the entity returned doesn't exist.

crates/bevy_winit/src/winit_windows.rs

@@ -281,7 +281,7 @@ impl WinitWindows {
     /// This should mostly just be called when the window is closing.
     pub fn remove_window(&mut self, entity: Entity) -> Option<winit::window::Window> {
         let winit_id = self.entity_to_winit.remove(&entity)?;
-        // Don't remove from `winit_to_window_id` so we know the window used to exist.
+        self.winit_to_entity.remove(&winit_id);
         self.windows.remove(&winit_id)
     }
 }