[Cin] small cleanup patch for filedv.h

according to my grep-fu this define not used anywhere, probably remnant from some cinelerra-cv era experiments in using ffmpeg for dv (today we can do the same directly) grep DV_USE_FFMPEG cinelerra/* grep: cinelerra/data: Это каталог cinelerra/filedv.h:#ifdef DV_USE_FFMPEG grep: cinelerra/i686: Это каталог grep: cinelerra/tests: Это каталог