By Laura McGuire Private equity firm Blackstone has abandoned plans to sell a 15-storey building in Canary Wharf in the latest signal that confidence in the east London financial district is dwindling. The world’s largest asset management firm was supposed to sell Cargo at 25 North Colonnade to an Asian investor late last year but the deal has since collapsed, Bloomberg reported. It is now understood the American giant will hold onto the 15 floor building, which is principally leased to BP, until sentiment in the market improves. Blackstone bought the tower, located one minute away from the Wh…