PM: Remove obsolete /sys/devices/.../power/state docs

The /sys/devices/.../power/state files have been gone for a while
now, but I just noticed some documentation that still refers to
them.  (Fortunately described as DEPRECATED and WILL REMOVE).

Time to remove that obsolete documentation too ...

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>