dt-bindings: iio: temperature: ltc2983: drop $ref for -nanoamp properties

Currently there are -nanoamp properties both with and without a $ref.
dt-schema has been modified to handle it as a standard unit, but the
change has been reverted since there were still occurrences of
-nanoamp properties with a $ref.
Remove this since it's the only occurrence left.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221115151653.393559-1-cosmin.tanislav@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>