staging: iio: light: isl29018: Remove non-standard sysfs attributes

This patch removes non-standard sysfs attributes range, range_available,
adc_resolution and adc_resolution_available. It also removes the
corresponding show and store functions.

This is in preparation for using standard IIO attributes in order to move
the code out of staging.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>