PM / Usermodehelper: Cleanup remnants of usermodehelper_pm_callback()

usermodehelper_pm_callback() no longer exists in the kernel. There are 2
comments in kernel/kmod.c that still refer to it.

Also, the patch that introduced usermodehelper_pm_callback(), #included
two header files: <linux/notifier.h> and <linux/suspend.h>. But these are
no longer necessary.

This patch updates the comments as appropriate and removes the unnecessary
header file inclusions.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>