progsig: Fix test remove rail action clearing signal programs.

This could be triggered by using shift, using the land info window, etc.

src/rail_cmd.cpp

@@ -645,7 +645,7 @@ CommandCost CmdRemoveSingleRail(TileIndex tile, DoCommandFlag flags, uint32 p1,
 
 			/* Charge extra to remove signals on the track, if they are there */
 			if (HasSignalOnTrack(tile, track)) {
-				CheckRemoveSignal(tile, track);
+				if (flags & DC_EXEC) CheckRemoveSignal(tile, track);
 				cost.AddCost(DoCommand(tile, track, 0, flags, CMD_REMOVE_SIGNALS));
 			}