ASoC: codecs: wcd937x: Drop unused buck_supply

Last user of wcd937x_priv->buck_supply was removed in
commit 216d0413 ("ASoC: codecs: wcd937x: Remove separate handling
for vdd-buck supply").

Fixes: 216d0413 ("ASoC: codecs: wcd937x: Remove separate handling for vdd-buck supply")
Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250526-b4-b4-asoc-wcd9395-vdd-px-fixes-v1-2-0b8a2993b7d3@linaro.org


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