Remove legacy .aud sound defaults from Common traits

While C&C-specific sound defaults might be acceptable for C&C-specific traits like MadTank and Chronoshiftable, for common, generic traits like Building they no longer are.

OpenRA.Mods.Common/Traits/ProductionParadrop.cs

@@ -25,7 +25,7 @@ namespace OpenRA.Mods.Common.Traits
 		[ActorReference(typeof(AircraftInfo))] public readonly string ActorType = "badr";
 
 		[Desc("Sound to play when dropping the unit.")]
-		public readonly string ChuteSound = "chute1.aud";
+		public readonly string ChuteSound = null;
 
 		[Desc("Notification to play when dropping the unit.")]
 		public readonly string ReadyAudio = null;