ACPI: delete unused acpi_evaluate_string()

Roel found a logic issue in the #if 0 acpi_evaluate_string():

-           || (element->type != ACPI_TYPE_BUFFER)
+           && (element->type != ACPI_TYPE_BUFFER)

delete the dead code.

pointed-out-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>