Streamline Land activity

Removed some redundant parameters, some redundant overloads
and made Land always consider LandAltitude relative to target.

OpenRA.Mods.Common/Traits/Cargo.cs

@@ -193,7 +193,8 @@ namespace OpenRA.Mods.Common.Traits
 
 				Unloading = true;
 				if (aircraft != null)
-					self.QueueActivity(new Land(self, true));
+					self.QueueActivity(new Land(self));
+
 				self.QueueActivity(new UnloadCargo(self, true));
 			}
 		}