Remove unassigned HTTP status codes

ecfae279ee89b4c05d74cd6741f132f440ae469f, which I think was intended solely as a formatting/documentation fix, actually added several HTTP status codes that are designated by the IANA as "Unassigned". We should once again exclude these from Rack's list, if for no other reason than because the `SYMBOL_TO_STATUS_CODE` mapping generated from `HTTP_STATUS_CODES` includes an `:unassigned` key that arbitrarily points to the last status code with this name.