ACPI: button: remove control method/fixed hardware distinctions

This patch removes the driver distinction between control method (CM)
and fixed hardware (FF) buttons.  We previously needed that so we
could install either a fixed event handler or a notify handler, but
the Linux/ACPI code now handles that for us, so we don't need to
worry about it.

Note that this removes the FF/CM annotation from the "info" files
in /proc.  For example,

    /proc/acpi/button/PWRF/info:
    -type:		Power Button (FF)
    +type:		Power Button

I don't think there's anything meaningful user-space can do by
knowing whether a button is a control method or a fixed hardware
button, so nobody should be looking at the FF/CM.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>