BERLIN — Western powers need to start planning for a Libya without Moammar Gaddafi because the Libyan leader’s departure from power could happen at any time, NATO’s head said Wednesday, as he gave a tacit endorsement to the rebel-led government in the country. Over the past two days, the military alliance has been engaged in the most intense bombardment of Tripoli since operations began in March, and the Libyan government has said that NATO is trying to assassinate Gaddafi.