Dissolve RA.Buildings namespace.

Move a handfull related, loose traits to RA.Traits as well.

OpenRA.Mods.RA/Traits/Buildings/Bridge.cs

@@ -14,7 +14,6 @@ using System.Linq;
 using OpenRA.Effects;
 using OpenRA.GameRules;
 using OpenRA.Graphics;
-using OpenRA.Mods.RA.Buildings;
 using OpenRA.Primitives;
 using OpenRA.Traits;