Fix: remove assertions of the existence of a trace chunk

A session can have no trace chunk on the session daemon and
consumer daemon end whenever its ouput is remote (relayd) and
doesn't support trace chunks.

The assertions removed as part of this patch are overly strict
considering that there are valid cases when trace_chunk can
be NULL.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>