matroskadec: partly revert "demux relevant subtitle packets after a seek"

This reverts parts of c1658257. The hard
coded 30 seconds are a lot, and finishing the seek can takes several
seconds when the source is on a network share. Remove this code
entirely, because it does more bad than good.

(Commit message provided by committer, based on the original messages
by the patch author.)

Signed-off-by: Rainer Hochecker <fernetmenta@online.de>
Signed-off-by: wm4 <nfxjfg@googlemail.com>