Remove inconsistencies w.r.t the Surrender order

* Surrendering is now possible even in maps that do not use
  ConquestVictoryConditions.
* The Surrender button is greyed out instead of hidden when it cannot be
  used.

OpenRA.Mods.RA/ConquestVictoryConditions.cs

@@ -21,7 +21,7 @@ namespace OpenRA.Mods.RA
 		public object Create(ActorInitializer init) { return new ConquestVictoryConditions(init.self, this); }
 	}
 
-	public class ConquestVictoryConditions : ITick, IResolveOrder, INotifyObjectivesUpdated
+	public class ConquestVictoryConditions : ITick, INotifyObjectivesUpdated
 	{
 		readonly ConquestVictoryConditionsInfo info;
 		readonly MissionObjectives mo;