Slay charge tossed BY PETER KADUSHIN and LEO STANDORA DAILY NEWS WRITERS A Mexican immigrant languished in a Rikers Island jail cell on murder charges for more than four years before Queens prosecutors finally admitted yesterday they had no case. Lourdes Torres had tears in her eyes when the charges were dismissed in Queens Supreme Court. "Thank you. I am grateful," said the 31-year-old woman, who, throughout her ordeal, denied she killed her boyfriend.