staging:iio:ad7606: Remove unused int_vref_mv field

Remove the int_vref_mv field from the ad7606_chip_info struct since the
field is never used by the driver. The value is also the same for all
derivatives of this chip, so if it will ever be used in the driver a
constant value will work just fine.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>