"maxim,max1237" is already documented in iio/adc/maxim,max1238.yaml, so
drop it from trivial-devices.yaml.

Link: https://lore.kernel.org/r/20240903-dt-trivial-devices-v1-4-ad684c754b9c@kernel.orgSigned-off-by: Rob Herring (Arm) <robh@kernel.org>