ACPI: delete obsolete "bus master activity" proc field

Linux-2.6.29 deleted the legacy ACPI idle handler, leaving
the CPU_IDLE handler, which does not track bus master activity.

So delete the unused bm_activity field -- it is confusing to
print an always zero value.

This patch could break programs that parse
/proc/acpi/processor/*/power, since it deletes this
line from that file:

bus master activity:     00000000

http://bugzilla.kernel.org/show_bug.cgi?id=13145


is not fixed by this patch, but provoked this patch.

Signed-off-by: Len Brown <len.brown@intel.com>