Fix: Make shipwreck sound entity despawn itself

I started to work on weapon fire sounds and realized I don't know if the
entity and/or component get removed after playback ends. It turns out
that they DO NOT!... unless told to do so, like this.

src/objects.rs

@@ -237,7 +237,7 @@ pub fn ship_impact_listener(
         // STEP 5: Play crash sound
         commands.spawn((
             AudioPlayer::new(game_assets.wreck_sound()),
-            PlaybackSettings::ONCE,
+            PlaybackSettings::DESPAWN, // despawn this entity when playback ends.
         ));
     }
 }