iio: adc: twl4030: Remove twl4030_madc_request.func_cb

This functionality is not used by the IIO subsystem. Due
to removal of legacy API it can also be removed.

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>