bt2: remove comment reference to "output port message iterator"

bt2: remove comment reference to "output port message iterator"

Output port message iterators is a concept that existed in preliminary
versions of Babeltrace 2, but did not exist in the final release.  This
comment is stale.

Change-Id: I012e640eb33fe1dbf84b3863ae21a313f6de57f3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3881
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>