ACPI: Fix unused variable warning in sbs.c

When CONFIG_ACPI_SYSFS_POWER=n and CONFIG_ACPI_PROCFS_POWER=n, then
we're warned by the following warning:

drivers/acpi/sbs.c: In function `acpi_battery_remove':
drivers/acpi/sbs.c:825: warning: unused variable `battery'

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>