Remove declaration of the removed function img_convert().

Originally committed as revision 18080 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.h

@@ -35,7 +35,4 @@ int ff_get_plane_bytewidth(enum PixelFormat pix_fmt, int width, int plane);
 
 int ff_set_systematic_pal(uint32_t pal[256], enum PixelFormat pix_fmt);
 
-int img_convert(AVPicture *dst, int dst_pix_fmt, const AVPicture *src,
-                int src_pix_fmt, int src_width, int src_height);
-
 #endif /* AVCODEC_IMGCONVERT_H */