iio: magnetometer: ak8974: remove incorrect __exit markups

iio: magnetometer: ak8974: remove incorrect __exit markups

Even if bus is not hot-pluggable, devices can be unbound from the
driver via sysfs, so we should not be using __exit annotations on
remove() methods. The only exception is drivers registered with
platform_driver_probe() which specifically disables sysfs bind/unbind
attributes.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>