This reverts commit 7360db05b4.
This commit was added as an attempt to fix#6046. Unfortunately, the fix
is completely wrong. As reported on #10359, embedded thumbnails are not
displayed in VLC, and Se7en on IRC reports that the embedded thumbnail
misleads mpv as well.
The correct way is using -attachment of ffmpeg, while the current
run_ffmpeg_multiple_files API can't handle it cleanly.