iio: sx9310: Drop channel_users[]

This struct member isn't used. Drop it.

Fixes: 72ad02b1 ("iio: Add SEMTECH SX9310/9311 sensor driver")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniel Campello <campello@chromium.org>
Signed-off-by: Daniel Campello <campello@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>