Fix: reduce cost of building canals over objects on sea

It is not like we will drain the sea first, to put water back in it after.
Besides, the cost for draining the sea isn't calculated for all other cases either.

src/water_cmd.cpp

@@ -479,7 +479,7 @@ CommandCost CmdBuildCanal(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 		if (IsTileType(current_tile, MP_WATER) && (!IsTileOwner(current_tile, OWNER_WATER) || wc == WATER_CLASS_SEA)) continue;
 
 		bool water = IsWaterTile(current_tile);
-		ret = DoCommand(current_tile, 0, 0, flags | DC_FORCE_CLEAR_TILE, CMD_LANDSCAPE_CLEAR);
+		ret = DoCommand(current_tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 		if (ret.Failed()) return ret;
 
 		if (!water) cost.AddCost(ret);