The fortunes of the housebuilding sector in the City resemble an Agatha Christie play as leading players are ejected one-by-one from the blue chip FTSE 100 index. Last year, as the industry rode out the end of the house price boom, Barratt, Persimmon and Taylor Wimpey were FTSE 100 fixtures. Not now. Barratt went first and Taylor Wimpey followed last …