LU-17139 utils: l_getidentity remove 'files' => 'lustre' alias

LU-17139 utils: l_getidentity remove 'files' => 'lustre' alias

Fully remove the old 'files' alias for 'lustre' and deprecate
'nss_files' alias for 'files' now that it can be used directly.

Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ied7cf87e9be8d474f9b65a9b5b14870578806151
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52487
Reviewed-by: corey tesdahl <corey.tesdahl@hpe.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>