avfilter/audio: Remove unused array, move used-only-once array

avfilter/audio: Remove unused array, move used-only-once array

ff_planar_sample_fmts_array is unused (and was unused since it was added
in 4d4098da009c8340997b8d1abedbf2062e4aa991) and therefore this commit
removes it; ff_packed_sample_fmts_array meanwhile is used only once (in
the amerge filter) and therefore it has been moved to this place.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>