PM: Update documentation regarding sysdevs

The part of Documentation/power/devices.txt regarding sysdevs is not
valid any more after commit 2e711c04dbbf7a7732a3f7073b1fc285d12b369d
(PM: Remove sysdev suspend, resume and shutdown operations), so
remove it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
1 file changed