ACPI: remove CONFIG_ACPI_SYSTEM

Remove CONFIG_ACPI_SYSTEM.  It was always set the same as CONFIG_ACPI,
and it had no menu label, so there was no way to set it to anything
other than "y".

Some things under CONFIG_ACPI_SYSTEM (acpi_irq_handled, acpi_os_gpe_count(),
event_is_open, register_acpi_notifier(), etc.) are used unconditionally
by the CA, the OSPM, and drivers, so we depend on them always being
present.

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