nfit/ars: Remove ars_start_flags

The ars_start_flags property of 'struct acpi_nfit_desc' is no longer
used since ARS_REQ_SHORT and ARS_REQ_LONG were added.

Reviewed-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>