Revert "oggdemux: Prevent seeks when _SCHEDULING_FLAG_SEQUENTIAL is set"

Revert "oggdemux: Prevent seeks when _SCHEDULING_FLAG_SEQUENTIAL is set"

This reverts commit 76647f2710d718e27f207b005956b7dba72c2d19.

Avoiding pull mode activation is a feature regression, and
demuxers should always use pull mode where that is possible,
e.g. if there's an upstream queue2 with a ring buffer or
a download buffer.

This patch made reverse playback no longer possible over http.

If the goal is to minimise seeks, then that can still be done
by making the demuxer behave differently in pull mode if
the SEQUENTIAL flag is set. If there are bugs, like the demuxer
needlessly scanning the entire file on start-up in pull mode,
then those should be fixed instead.

https://bugzilla.gnome.org/show_bug.cgi?id=746010