Former Los Angeles County Dist. Atty. Robert H. Philibosian said he will not be a Republican candidate against state Atty. Gen. John Van de Kamp. After exploring the possibility for several months, Philibosian sent his supporters letters saying he had "too many other priorities." Philibosian, now in private law practice, said he will concentrate on reelecting Gov. George Deukmejian, and assisting in the campaign to defeat Chief Justice Rose Elizabeth Bird.