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

Documentation/devicetree/bindings/iio/temperature/adi,ltc2983.yaml

@@ -325,7 +325,6 @@ patternProperties:
         description:
           Excitation current applied to the thermistor.
           0 sets the sensor in auto-range mode.
-        $ref: /schemas/types.yaml#/definitions/uint32
         enum: [0, 250, 500, 1000, 5000, 10000, 25000, 50000, 100000, 250000,
                500000, 1000000]
         default: 0