mpegts: Always clear packetizer on DISCONT push mode

mpegts: Always clear packetizer on DISCONT push mode

If a discontinuity is detected in push mode, we need to clear the cached section
observations since they might have potentially changed.

This was only done properly when operating with TIME segments (dvb, udp,
adaptive demuxers, ...) but not with BYTE segments (such as with custom app/fd
sources).

We still don't want to flush out the PCR observations, since this might be
needed for seeking in push-based BYTE sources.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1650

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3584>