According to the Daily Mail, William and Harry, who are 6’2″ and 6’1″ respectively, were too tall as stormtroopers, who are supposed to be exactly 5’11”. It sounds like it was expected that the shot would make it into the movie, but ultimately it did not. The scene the two were supposed to appear in would have taken place in the latter half of the film aboard the Supremacy. While Finn, Rose, BB-8 and DJ are infiltrating the First Order ship they would have found themselves inside an elevator, along with some stormtroopers who were being played by the princes. In the scene one of the stormtroopers would have slapped DJ on the butt, for reasons that remain unclear. Apparently the scene did not work for the final film, so it was cut.