A charge of attempted murder was dismissed against Van Nuys physician John Frederick Kappler, whom authorities had accused of removing, without authorization, the life-support system of a brain-damaged patient at Hollywood Presbyterian Medical Center last April. A Los Angeles Municipal Court judge decided after a two-day preliminary hearing that prosecutors were unable to provide witnesses who could positively identify Kappler as having been in the hospital room of the patient when the respirator was turned off. The patient was stabilized by nurses and suffered no unusual complications. Deputy Dist. Atty. David P. Conn had no comment on whether the case will be appealed or refiled.