Remove documentation for now-removed power restore behavior configuration

[tikkamasala2]

#156 tracked adding documentation for /sys/bus/platform/devices/macsmc-reboot/ac_power_mode for controlling power restore behavior, this functionality was first introduced here AsahiLinux/linux@233aeaf but looks like it was removed as part of this change: AsahiLinux/linux@819687e

This should be removed from the docs here to avoid confusion.