The dawg days are over at "American Idol" ... TMZ has learned Randy Jackson is officially LEAVING the judge's table ... but will still remain a part of the show as a "mentor." Sources said... there's some concern among the brass that Jackson is not well-suited for the mentor role ... but they feel they need him because he's the glue that helps keep the show together.  As for his replacement ... sources tell us Nicki Minaj's deal is 99% done ... but still not a sure thing just yet. We're told the honchos are keenly aware Mariah Carey isn't a fan of Minaj and doesn't want her at the table ... a fact producers LOVE because they think the conflict and tension will be good for the show.