staging:iio:events: Remove obsolete documentation

Commit 43ba1100 ("staging:iio:events: Use waitqueue lock to protect event
queue") removed the event_list_lock field from the iio_event_interface struct,
but missed to remove the same field from the documentation for that function.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>