sink.utils.dummy: remove unnecessary check in dummy_consume()

sink.utils.dummy: remove unnecessary check in dummy_consume()

Given the lifecycle of a component, I'm pretty sure this condition can
never be true.  For dummy_consume() to be called,
dummy_graph_is_configured() must execute successfully.  If
dummy_graph_is_configured() executes successfully, `dummy->msg_iter` is
necessarily set.

Change-Id: I5c11c6b76c8cb267f0185c1a35bc2f5e70826509
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12009
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>