Revert "avcodec/adpcm_swf: support decoding multiple fixed-sized blocks at once"

Revert "avcodec/adpcm_swf: support decoding multiple fixed-sized blocks at once"

Is incorrect behaviour. Was covering for an encoder bug where it produced frames
of the wrong size.

This reverts commit e9dd73d30d09043446ac6dd7b8ad31e557873852.

Fixes: out of array write
Fixes: 26821/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_SWF_fuzzer-5764465137811456
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Zane van Iperen <zane@zanevaniperen.com>