doc/libav-merge: remove line about VP9 superframe parsing

It was addressed in a5679933,
0c162854 and
5a366f97.
Signed-off-by: James Almer <jamrial@gmail.com>

doc/libav-merge.txt

@@ -97,7 +97,6 @@ Stuff that didn't reach the codebase:
   - 7993ec19a hevc: Add hevc_get_pixel_4/8/12/16/24/32/48/64
 - Removal of the custom atomic API (5cc0057f49, see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-March/209003.html)
 - new bitstream reader (see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-April/209609.html)
-- use of the bsf instead of our parser for vp9 superframes (see fa1749dd34)
 - use av_cpu_max_align() instead of hardcoding alignment requirements (see https://ffmpeg.org/pipermail/ffmpeg-devel/2017-September/215834.html)
   - f44ec22e0 lavc: use av_cpu_max_align() instead of hardcoding alignment requirements
   - 4de220d2e frame: allow align=0 (meaning automatic) for av_frame_get_buffer()