PM: Update documentation regarding sysdevs

The part of Documentation/power/devices.txt regarding sysdevs is not
valid any more after commit 2e711c04
(PM: Remove sysdev suspend, resume and shutdown operations), so
remove it.
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>

Documentation/power/devices.txt

@@ -549,32 +549,6 @@ callbacks.  The other platforms need not implement it or take it into account
 in any way.
 
 
-System Devices
---------------
-System devices (sysdevs) follow a slightly different API, which can be found in
-
-	include/linux/sysdev.h
-	drivers/base/sys.c
-
-System devices will be suspended with interrupts disabled, and after all other
-devices have been suspended.  On resume, they will be resumed before any other
-devices, and also with interrupts disabled.  These things occur in special
-"sysdev_driver" phases, which affect only system devices.
-
-Thus, after the suspend_noirq (or freeze_noirq or poweroff_noirq) phase, when
-the non-boot CPUs are all offline and IRQs are disabled on the remaining online
-CPU, then a sysdev_driver.suspend phase is carried out, and the system enters a
-sleep state (or a system image is created).  During resume (or after the image
-has been created or loaded) a sysdev_driver.resume phase is carried out, IRQs
-are enabled on the only online CPU, the non-boot CPUs are enabled, and the
-resume_noirq (or thaw_noirq or restore_noirq) phase begins.
-
-Code to actually enter and exit the system-wide low power state sometimes
-involves hardware details that are only known to the boot firmware, and
-may leave a CPU running software (from SRAM or flash memory) that monitors
-the system and manages its wakeup sequence.
-
-
 Device Low Power (suspend) States
 ---------------------------------
 Device low-power states aren't standard.  One device might only handle