ad_ffmpeg: remove incorrect request_sample_fmt setting
Commit
91a18f3998 ("audio, libav: support planar Libav audio formats")
added a line setting request_sample_fmt to AV_SAMPLE_FMT_S16P. This
broke FLAC decoding with libavcodec versions since July but without
libavresample (in July the FLAC decoder got the ability to support
either planar or interleaved, depending on request_sample_fmt;
requesting planar without libavresample causes failure). According
to the author this line was left over from testing. Remove.