lib: remove TODO in add_port

I think that the {input,output}-port-name-is-unique assertions basically
check that this TODO is about.

Change-Id: Ib5029697114f46fd5485d902068ca912b658ebc9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10038
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>