cli: `convert` command: do not allow more than one sink component

cli: `convert` command: do not allow more than one sink component

It's a nonsense to allow more than one component sink because, starting
from the muxer's single output port, there's a chain of components until
it reaches a sink. We don't have any tee filter, so we can't have more
than one sink: the second one won't get connected.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ibec54ae29b64bb0fb8637f1b37131f05abc53b77
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1953
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>