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.
