This does not fix anything but gives users a chance to
know that they must not pass -sub_charenc UTF-16 to ffmpeg.

Fixes ticket #4059.

Also remove ff_smil_extract_next_chunk - this was the last user of it.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Demuxers where the changed code looked less readable where skiped
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

This flag is not necessary.