Clear messed up feature flag on AppExitStates impl (bevyengine#13737)

# Objective

- In bevyengine#13649 additional method had been added to AppExitStates, but there
feature gate left for method in implementation for App at refactoring
stage.
- Fixes bevyengine#13733 .

## Solution

- Removed the feature gate.

## Testing

- Ran reproducing example from bevyengine#13733 with no compilation errors

crates/bevy_state/src/app.rs

@@ -177,7 +177,6 @@ impl AppExtStates for App {
  self
  }
 
- #[cfg(feature = "bevy_hierarchy")]
  fn enable_state_scoped_entities<S: States>(&mut self) -> &mut Self {
  self.main_mut().enable_state_scoped_entities::<S>();
  self