iio: st_sensors: remove all driver remove functions

iio: st_sensors: remove all driver remove functions

At this point all ST driver remove functions do iio_device_unregister().
This change removes them from them and replaces all iio_device_register()
with devm_iio_device_register().

This can be done in a single change relatively easy, since all these remove
functions are define in st_sensors.h.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210823112204.243255-5-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>