Fix confusing comment in pbr example (#16996)

After a recent fix for a panic in the pbr example (#16976), the code
contains the following comment:

```rust
// This system relies on system parameters that are not available at start
// Ignore parameter failures so that it will run when possible
.add_systems(Update, environment_map_load_finish.never_param_warn())
```

However, this explanation is incorrect. `EnvironmentMapLabel` is
available at start. The real issue is that it is no longer available
once it has been removed by `environment_map_load_finish`.

- Remove confusing/incorrect comment and `never_param_warn()`.
- Make `Single<Entity, With<EnvironmentMapLabel>>` optional in
`environment_map_load_finish`, and check that the entity has not yet
been despawned.

Since it is expected that an entity is no longer there once it has been
despawned, it seems better to me to handle this case in
`environment_map_load_finish`.

Ran `cargo run --example pbr`.

examples/3d/pbr.rs

@@ -7,9 +7,7 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)
-        // This system relies on system parameters that are not available at start
-        // Ignore parameter failures so that it will run when possible
-        .add_systems(Update, environment_map_load_finish.never_param_warn())
+        .add_systems(Update, environment_map_load_finish)
         .run();
 }