iio: adc: twl4030: Drop twl4030_get_madc_conversion()

Drop legacy twl4030_get_madc_conversion() method. It has been
used by drivers to get madc data before it conversion to IIO
API. There are no users in the mainline kernel anymore.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>