ACPI: remove /proc/acpi/event from ACPI_BUTTON help

Commit 1696d9dc (ACPI: Remove the old /proc/acpi/event interface)
left /proc/acpi/event in the ACPI_BUTTON help in Kconfig, so
remove it from there.

[rjw: Changelog]
Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>