bt_common_parse_lttng_live_url(): don't accept `net://RDHOST/host/TGTHOST`

bt_common_parse_lttng_live_url(): don't accept `net://RDHOST/host/TGTHOST`

This patch makes the bt_common_parse_lttng_live_url() function not
accept LTTng live URLs with the form:

    net[4]://RDHOST[:RDPORT]/host/TGTHOST

This was previously accepted for the session listing feature, but in
fact the `babeltrace` program (Babeltrace 1) only accepts this form:

    net[4]://RDHOST[:RDPORT]

and the list of URLs that the `babeltrace2` program prints with the
first form is invalid.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I68a804c69ec557ed9cc77c9020afbbe2f79a7c43
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1961
Tested-by: jenkins <jenkins@lttng.org>