TEHRAN A grand ayatollah yesterday said that Imam Khomeini's fatwa on apostate Salman Rushdie is irrevocable. Imam Khomeini issued a fatwa on Salman Rushdie in 1989 condemning him to death for blaspheming the Prophet of Islam in his book The Satanic Verses. Iran gave assurance to Britain Thursday that the government of the Islamic Republic of Iran will not implement the fatwa and reiterated that the fatwa was irrevocable.