Producers were forced to cancel the Sunday matinee performance of the hit Broadway revival of Arthur Miller's "Death of a Salesman," because its star, Brian Dennehy, had been hospitalized for hypertension. Dennehy, who has no understudy in the classic role of Willy Loman, was rushed to the hospital early Sunday morning, after feeling sick and sleepless following Saturday night's show at the Eugene O'Neill Theater. The 59- year-old actor was transferred Sunday from St. Clare's Hospital to the cardiac unit at St. Vincent's Medical Center in Greenwich Village. Dennehy, considered the front-runner for this year's Best Actor Tony, is expected to return to the show Wednesday night.