iio: adc: mxs-lradc: Remove driver

Since the driver has been split into MFD there is no reason for it to
stay, so remove it.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>