Exxon Mobil Corp.’s Canadian unit Imperial Oil Ltd. is scaling back the amount of U.S.-bound oil it ships by rail from the province of Alberta to nearly zero this month. The company shipped an average of roughly 90,000 barrels a day by rail to U.S. refineries in January, Imperial Chief Executive Richard Kruger said during a conference call Friday. A recent surge in the price of western Canadian heavy crude—fueled by an Alberta-mandated production cut unveiled in December—means it has become too expensive to ship that oil…