pm: remove obsolete piece of PM documentation

Remove some obsolete PM documentation.

The majority of contents of Documentation/power/pm.txt are
outdated.  Remove the outdated parts of this file and move the rest
to Documentation/power/apm-acpi.txt .  Update the index in
Documentation/power/ as appropriate.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>