dt-bindings: iio: adc: drop redundant type from label

dtschema defines label as string, so $ref in other bindings is
redundant.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240226123004.91061-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adc.yaml

@@ -22,7 +22,6 @@ properties:
     maxItems: 1
 
   label:
-    $ref: /schemas/types.yaml#/definitions/string
     description: Unique name to identify which channel this is.
 
   bipolar:

Documentation/devicetree/bindings/iio/adc/qcom,spmi-vadc.yaml

@@ -75,7 +75,6 @@ patternProperties:
           in the PMIC-specific files in include/dt-bindings/iio/.
 
       label:
-        $ref: /schemas/types.yaml#/definitions/string
         description: |
             ADC input of the platform as seen in the schematics.
             For thermistor inputs connected to generic AMUX or GPIO inputs