drivers/iio/industrialio-event.c: eliminate possible double free
The function __iio_add_event_config_attrs is only called once, by the
function iio_device_register_eventset. If the call fails,
iio_device_register_eventset calls __iio_remove_event_config_attrs. There
is thus no need for __iio_add_event_config_attrs to also call
__iio_remove_event_config_attrs on failure.
A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)