Deutsche Bank agreed on Thursday to sell Cosmopolitan Casino in Las Vegas, which is one of the town’s most expensive resorts. The property will be sold to the Blackstone Group for about $1.73 billion in cash. The deal will finally end a long and painful foray by Deutsche Bank into the casino business, after it foreclosed on the resort’s developer, Bruce Eichner, in January 2008. It was one of many, as banks moved to take over troubled gambling properties at the onset of the financial crisis. Since then, the firm has tried to make the best of its new property,…