iio: adc: twl4030: Drop struct twl4030_madc_user_parms

This struct is no longer used by anything in the kernel.

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>