(Reuters) - NMC Health said on Friday Executive Vice Chairman Khalifa Butti Omeir Bin Yousef had resigned from its board, the latest sign of turbulence at the United Arab Emirates largest private healthcare provider as it eyes a sale to one of two private equity groups.On Monday, the company's co-chair and founder B.R. Shetty said he was stepping back from board functions after he informed NMC of a potentially inaccurate reporting of his holding in the company. The holdings of Shetty, Khalifa..