By Mohammad Yunus Yawar, Idrees Ali and Jeff Mason KABUL/WASHINGTON (Reuters) -The United States killed al Qaeda leader Ayman al-Zawahiri with a drone missile while he stood on a balcony at his home in Kabul, U.S. officials said, the biggest blow to the militants since Osama bin Laden was shot dead more than a decade ago. Afghanistan’s Taliban government has not confirmed the death of Zawahiri, an Egyptian surgeon who had a $25 million bounty on his head and helped to coordinate the Sept. 11, 2001, attacks on the United States that killed nearly 3,000 people. U.S. officials, speaking on the co…