avformat/vobsub: do not attempt to check duplicate subtitles
Some .idx files actually contain duplicate subtitle events:
timestamp: 00:07:52:600, filepos:
00004e800
timestamp: 00:07:52:600, filepos:
00004f800
The second will be dropped, because it has same pts, duration, and text
(the text is just a dummy empty string; the real data is retrieved when
actually reading vobsub subtitle packets).
Dropping this is probably not intended/safe, so avoid it.
See trac issue #4872 for a sample. This patch doesn't fix decoding of
the sample, though.