iio: mxs-lradc: remove mxs_lradc_ts_unregister

After using devm_input_allocate_device for registration
the function mxs_lradc_ts_unregister isn't necessary anymore
since mxs_lradc_ts_close already does the job.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>