sink.utils.counter: remove unnecessary check in counter_consume()

sink.utils.counter: remove unnecessary check in counter_consume()

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

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ibd3cbcfdeefa8d79143e39c6fb846ca5eeebd249
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12010
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>