hwmon: (pmbus) remove CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default. As agreed during the Linux kernel
summit, remove it.

CC: Guenter Roeck <linux@roeck-us.net>
CC: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>