THE MAN ONCE DUBBED ""Obsequio'' for his fawning demeanor won't have to worry about that nickname anymore. In his final week on the air, Arsenio Hall invited media execs to ""kiss my black ass'' and stridently told ""Empty Nest'' star Park Overall that racism rages unabated. What gives? Sources say Hall thinks Paramount deserted him in the face of stiff new competition. ""For a brief minute, he felt he would be allowed to play ball with the white boys and get his fair chance at bat,'' says one Hollywood pal. ""The Leno-Letterman situation let him know real quick he was just another ball boy.'' Says another: ""He tried to please The Man, and The Man still jacked him in the end, so now he knows.''