Prince William and King Charleshave become each other’s biggest support system as His Majesty and Kate Middleton battle cancer. According to a source, the king and his heir are “closer than ever” as they juggle leading The Crown and the Princess of Wales’ medical leave. “If there was ever a green-eyed monster or a sense of rivalry between the two, that is a chapter of the past,” the source told an outlet. “The King sees his son as a useful ally on family matters and increasingly in discharging the duties of nation and state.” “He very much wants to support his father,” they added. “That isn’t …