hwmon: (twl4030-madc) drop driver

This driver is no longer needed:

 * It has no mainline users
 * It has no DT support and OMAP is DT only
 * iio-hwmon can be used for madc, which also works with DT

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>