staging: iio: tsl2x7x: remove unnecessary sysfs attribute

staging: iio: tsl2x7x: remove unnecessary sysfs attribute

The tsl2771 and tmd2771 devices create the
in_proximity0_calibscale_available sysfs attribute. These two particular
devices do not support changing the proximity gain value on the
chip so this patch removes that sysfs attribute. As expected, these two
devices already did not create the IIO_CHAN_INFO_CALIBSCALE channel and
proximity0_calibrate sysfs attribute.

Page 38 of the tsl2772 data sheet shows that the proximity gain can be
adjusted with bits 2-3 on the control register:
https://ams.com/eng/content/download/291503/1066377/file/TSL2772_DS000181_2-00.pdf

Page 35 of the tsl2771 and tmd2771 data sheets shows that bits 2-3 on
the control register are reserved and changing the proximity gain is
not supported:
https://ams.com/eng/content/download/250264/976045/file/TSL2771_DS000105_3-00.pdf
https://ams.com/eng/content/download/250283/976077/file/TMD2771_DS000177_2-00.pdf

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>