Citigroup‘s board said on Tuesday that Vikram S. Pandit had stepped down as chief executive, effective immediately, and will be succceeded by the head of the bank’s European and Middle Eastern division, Michael L. Corbat. John P. Havens, the bank’s president and a longtime associate of Mr. Pandit’s, has also resigned. The surprising move by Mr. Pandit comes just one day after the firm reported stronger-than-expected third-quarter earnings. Excluding a number of onetime charges — including a big loss tied to the continued shedding of the Smith Barney brokerage — Citigroup earned $3.27 billion, or $1.06 a share. That bested...