ACPI: AC: Remove the leftover struct acpi_ac_bl

In commit 57a18322 ("ACPI / x86: Introduce an
acpi_quirk_skip_acpi_ac_and_battery() helper"), the usage of struct
acpi_ac_bl was removed, but left the definition of the struct in
the file, so remove the useless code.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>