With a winter storm blanketing a large swath of the country, Hollywood had its quietest weekend of the year at the box office. The Amazon MGM sci-fi thriller “Mercy” dethroned “Avatar: Fire and Ash” from the No. 1 spot with $11.2 million in North America, according to studio estimates Sunday. Hundreds of theaters were forced to close due to the storm spreading from Texas to Maine. But the weekend was never going to be a blockbuster one. “Mercy,” starring Chris Pratt, arrived with withering reviews and an equally poor reception from audiences. But it was still enough to finally dislodge James Cameron’s third Pandora epic from its monthlong perch atop the box office.