ACPI: Remove the old /proc/acpi/event interface

It is quite some time that this one has been deprecated.
Get rid of it.

Should some really important user be overseen, it may be reverted and
the userspace program worked on first, but it is time to do something
to get rid of this old stuff...

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Matthew Garrett <matthew.garrett@nebula.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>