Joss Whedon has exited the Warner Bros’ Batgirl, a DC stand-alone movie that he came aboard last year to write, direct and produce. The deal was first made public last March, and would have seen the writer and director of Disney/Marvel’s tentpole blockbuster The Avengers join a rival comics universe of movies. He said today essentially he couldn’t crack the story of the Gotham heroine, who as been in DC Comics lore since the late 1960s. "Batgirl is such an exciting…