iio: core: fix ioctl handlers removal

In the Linux kernel, the following vulnerability has been resolved:

iio: core: fix ioctl handlers removal

Currently ioctl handlers are removed twice. For the first time during

iio_device_unregister() then later on inside

iio_device_unregister_eventset() and iio_buffers_free_sysfs_and_mask().

Double free leads to kernel panic.

Fix this by not touching ioctl handlers list directly but rather

letting code responsible for registration call the matching cleanup

routine itself.