avformat: Deprecate AVFMT_FLAG_PRIV_OPT, remove av_demuxer_open on bump

This flag was added in 49202620
in conjunction with av_demuxer_open() to allow to pass private
options to demuxers. It worked as follows: av_open_input_stream()
(the predecessor of avformat_open_input()) would not call the
read_header function if this flag is set. Instead the user could set
private options of the demuxer via the format's private class after
avformat_open_input() and then call av_demuxer_open() which called
the format's read_header function.

This approach was abandoned in e37f161e
and av_demuxer_open() deprecated; instead the AVDictionary based way of
passing private options to the demuxer was choosen. Yet
AVFMT_FLAG_PRIV_OPT has never been deprecated and av_demuxer_open()
never removed. This commit implements the deprecation of the flag and
schedules av_demuxer_open for removal on the next major bump.
Given that av_demuxer_open() has been deprecated in 2012 and that this
flag is useless without it, the flag will be ignored after the next
major version bump.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>