ACPI / AC: Remove initializer for unused ident dmi_system_id

The ac.c code does not use the dmi_system_id ident member, so there is
no need to initialize it. This saves us storing the unused "thinkpad e530"
string as const data.

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