demux_mkv: don't probe start time by default

It isn't all that reliable, and improving it would make startup slower
and require more complexity. There isn't even a good reason to do this
(other than semi-broken mkv files), so don't do it. Also see previous
commit.

demux/demux_mkv.c

@@ -236,7 +236,6 @@ const struct m_sub_options demux_mkv_conf = {
         .subtitle_preroll = 2,
         .subtitle_preroll_secs = 1.0,
         .subtitle_preroll_secs_index = 10.0,
-        .probe_start_time = 1,
     },
 };