lavfi: remove unnecessary inclusion of libavcodec/avcodec.h in avfilter.h

lavfi: remove unnecessary inclusion of libavcodec/avcodec.h in avfilter.h

libavfilter API was designed in order to be clarly distinguished from the
libavcodec API, including avcodec.h in avfilter.h is not going to help to
stick to this principle.

The inclusion of libavutil/audioconvert.h in many files was required
because avcodec.h includes audioconvert.h.

libavfilter/avcodec.h is where the lavc/lavfi interface should be
entirely placed.