Balmain, a 66-year-old French fashion house that recently engineered a comeback with skimpy dresses and spray-on jeans that were shockingly priced, announced on Wednesday that its star designer, Christophe Decarnin, had left the company. His departure is the latest in a string of shake-ups that suggest, after the John Galliano debacle at Dior, that luxury fashion houses are becoming less interested in promoting lively personalities than in protecting their own brands.