[Cin] small cleanup patch for filedv.h

[Andrew Randrianasulu]

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
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