hw/acpi: remove unnecessary variable acpi_table_builtin

acpi_table_builtin is now always false, it is not necessary to check it
again.

This patch just removes it.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190214084939.20640-4-richardw.yang@linux.intel.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>