ASoC: dt-bindings: stm32: dfsdm: remove stm32-adfsdm.txt binding

Device tree audio configuration for STM32 DFSDM is already
covered in the following binding:
Documentation/devicetree/bindings/iio/adc/st,stm32-dfsdm-adc.yaml
Remove stm32-adfsdm.txt obsolete binding.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201020155709.2621-3-olivier.moysan@st.com


Signed-off-by: Mark Brown <broonie@kernel.org>