iio: adc: stm32: Drop unnecessary DT property presence check

There's no reason to check for regulator supply property presence before
calling devm_regulator_get_optional() as that will return -ENODEV if
the supply is not present.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Tested-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://patch.msgid.link/20250109182325.3973684-2-robh@kernel.org


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