iio: mxs-lradc: remove useless scale_available files

in_voltage8_scale_available and in_voltage9_scale_available are exposed to
userspace but useless as in_voltage8_raw and in_voltage9_raw are not available.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>