Fix: src.ctf.lttng-live: removing trace when not all streams are done

Fix: src.ctf.lttng-live: removing trace when not all streams are done

The following commit wrongfully added the removal of a live trace
object:
  commit c28512ab93b16501a8b0da494f0a03e5f0f22662
  Author: Francis Deslauriers <francis.deslauriers@efficios.com>
  Date:   Mon Oct 28 14:49:08 2019 -0400

      src.ctf.lttng-live: make `lttng_live_get_one_metadata_packet()` return status

The live trace objects should only be removed once all its stream
iterator have ended. We do that in next_stream_iterator_for_session()
function.

So, roll back this change but keep the switch statement so it's clearer
what are the possible outcomes of the preceding function call.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I7ade43a256b731563eec2b1930724edbaeefd122
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2350
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>