sink.text.details: write discarded events/packets CS props when supported

sink.text.details: write discarded events/packets CS props when supported

It is known that, for a given stream class, when discarded events and
packets are not supported, they also don't have default clock snapshots.

This patch makes the output go from:

    Trace class:
      Stream class (ID 0):
        Packets have beginning default clock snapshot: Yes
        Packets have end default clock snapshot: Yes
        Supports discarded events: No
        Discarded events have default clock snapshots: No
        Supports discarded packets: No
        Discarded packets have default clock snapshots: No

to:

    Trace class:
      Stream class (ID 0):
        Packets have beginning default clock snapshot: Yes
        Packets have end default clock snapshot: Yes
        Supports discarded events: No
        Supports discarded packets: No

so as to remove some noise.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib8c32a2dcad1ea51fedb37ca8a73d7255df0972e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1651
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>