After a brilliant career that pitted the undersized quarterback Doug Flutie against all types of critisism, only to have the quarterback prove the naysayers wrong time and time again, Doug Flutie expected to retire from pro football. Flutie?s exit from the game, widely expected and reported on Channel 4 last night, ends one storybook career for the Natick resident. He most likely will embark on a sports broadcastng career. New Enlgand Patriots coach Bill Belichick was noncommittal yesterday when asked about Flutie?s future. ?You?ll have to talk to Doug about what Doug?s future is,? he said. ?I won?t speak for Doug.?