iio: mxs: Remove unused struct mxs_lradc_chan

iio: mxs: Remove unused struct mxs_lradc_chan

This structure definition seems to be some kind of remnant from
previous development that accidentally made it mainline. Remove
it as it's unused.

Remove unused "enable" field from struct mxs_lradc as well. This
seems to be remnant of early development too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>