LONDON (Reuters) -Sterling held on to its gains on Thursday after Boris Johnson said he was quitting as prime minister following a rush of ministerial resignations and calls for him to go. The gains versus the dollar and the euro began after media reports of his imminent resignation earlier in the day. But the moves were small, with Johnson’s departure already seen as almost a certainty and largely priced in by markets. UK assets have largely reacted calmly to Johnson’s declining fortunes this week, with the deteriorating global economic backdrop viewed as far more important even if some immed…