The Harlem Gospel Choir has backed out of a holiday performance with controversial conservative talk-show host Glenn Beck, the Daily News has learned. The famous choir, which has performed for Nelson Mandela and Pope John Paul II, was set to appear in the simulcast film of Beck's novel "The Christmas Sweater - A Return to Redemption," which opens Thursday in theaters nationwide. The choir canceled the appearance Monday, citing financial reasons. James Rucker, executive director of Color for Change - which has helped persuade more than 80 advertisers to ditch Beck's show - said the group did an about-face after...