hwmon: (pmbus) shrink code and remove pmbus_do_remove()

The only action currently performed in pmbus_do_remove() is removing the
debugfs hierarchy. We can schedule a devm action at probe time and remove
pmbus_do_remove() entirely from all pmbus drivers.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20201026105352.20359-1-brgl@bgdev.pl
[groeck: Removed references to pmbus_do_remove from documentation]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>