LONDON (Reuters) - British supermarket group Sainsbury's said on Wednesday that Roger Davis, chairman of its poorly performing banking division, will step down.A search to replace Davis, who has chaired the bank for nearly seven years, will begin and a further announcement regarding timings of his retirement and news of his successor will be made in due course, it said.The announcement comes five months after Sainsbury's completed a strategic review of its financial services business, which..